Systems and methods for increasing engine power output under globally stoichiometric operation

ABSTRACT

Methods and systems are provided for increasing engine power via partial engine enrichment and exhaust gas recirculation. In one example, a method may include enriching a first set of engine cylinders, enleaning a second set of the engine cylinders, and operating a third set of the engine cylinders at stoichiometry, exhaust gas from all of the engine cylinders producing a stoichiometric mixture at a downstream emission control device, and providing exhaust gas recirculation (EGR) to an intake passage of the engine from the first set of cylinders. In this way, cooling effects from the partial enrichment and the EGR enable engine air flow, and thus engine power, to be increased while an efficiency of the emission control device is maintained, thereby decreasing vehicle emissions.

FIELD

The present description relates generally to methods and systems forcontrolling a vehicle engine to increase power output via exhaust gasrecirculation and air-fuel ratio adjustments.

BACKGROUND/SUMMARY

A typical spark ignition engine of a vehicle operates at stoichiometryduring most operating conditions, where it is supplied with just enoughfuel to react with an amount of air consumed by the engine. Operating atstoichiometry increases an efficiency of a three-way catalyst positionedin an exhaust system of the engine, thereby reducing vehicle emissions.In general, higher cylinder air charges increase engine power, and sosome engines are equipped with a turbocharger to harness heat from theexhaust gas to supply more air to the engine to increase engine power.However, increasing engine air flow increases exhaust system componenttemperature, including a temperature of a turbine of the turbochargerand a temperature of the three-way catalyst. Such temperature increasesmay degrade the turbine and the three-way catalyst, for example.

Therefore, typical spark ignition engines may employ a global enrichmentstrategy to increase engine power, where more fuel is supplied than fora complete reaction with the amount of air consumed by the engine. Inparticular, the additional, unreacted fuel cools exhaust systemcomponents, including the turbine and the three-way catalyst. Thisenables more air flow for increased power while reducing heat-relateddegradation to the downstream components compared with operating atstoichiometry. However, deviating from stoichiometry decreases anefficiency of the three-way catalyst, resulting in increased vehicleemissions.

The inventors herein have recognized that a solution is needed tomaintain or approach the power gains available through global enrichmentwhile reducing vehicle emissions. For example, increasingly stringentvehicle emissions standards may not enable such global enrichmentstrategies to be performed. Without the cooling effects of unreactedfuel achieved through enrichment, engine air flow may be reduced inorder to protect exhaust system components from heat-relateddegradation, thereby reducing a maximum achievable engine power.

In one example, the issues described above may be addressed by a method,comprising: while operating an engine, enriching a first set ofcylinders, enleaning a second set of cylinders, and maintaining a thirdset of cylinders at stoichiometry, exhaust gas from the first set, thesecond set, and the third set producing a stoichiometric mixture at adownstream emission control device, and providing exhaust gasrecirculation (EGR) to an intake passage of the engine from the firstset of cylinders. In this way, engine power output may be increased bypartially enriching the engine without increasing vehicle emissions.

As one example, the enriching the first set of cylinders, enleaning thesecond set of cylinders, and maintaining the third set of cylinders atstoichiometry may be responsive to an engine torque demand that isgreater than a threshold torque. As another example, additionally oralternatively, the enriching the first set of cylinders, enleaning thesecond set of cylinders, and maintaining the third set of cylinders atstoichiometry may be further responsive to a rate of the EGR reaching athreshold rate. For example, the threshold torque may correspond to atorque level above which continued torque increases via increased airflow while operating each cylinder of the engine at stoichiometry mayincrease an exhaust temperature above a threshold temperature and abovewhich providing EGR alone is unable to maintain the exhaust temperaturebelow the threshold temperature.

As another example, the engine may include an odd number of cylinders.Therefore, the third set of cylinders may include one cylinder, thefirst set of cylinders may include a first half of a total number ofremaining cylinders in the engine, and the second set of cylinders mayinclude a second half of the total number of remaining cylinders in theengine. Further, an EGR passage may be coupled between an exhaust runnerof at least one cylinder of the first set of cylinders and the intakepassage in order to preferentially provide EGR from the first set ofcylinders. By providing EGR from the first set of cylinders, the EGR mayalso be enriched. The enriched EGR may provide knock suppression due tohigher concentrations of hydrogen gas and carbon monoxide, enablingspark timing to be advanced for further increasing the engine torque. Assuch, the spark timing may be individually adjusted for each cylinder inorder to account for different burn rates of the first (enriched) set ofcylinders, the second (enleaned) set of cylinders, and the third(stoichiometric) set of cylinders.

By partially enriching the engine while maintaining an overall air-fuelratio of the exhaust gas at stoichiometry, the exhaust system componentcooling effects of enrichment may be achieved while the emission controldevice is operated efficiently, thereby decreasing vehicle emissionswhile engine power is increased. Further, the enriched EGR may provideadditional exhaust system component cooling as well as knocksuppression, enabling spark timing to be optimized for a furtherincrease in engine power. Overall, the engine may be operated withgreater air consumption for increased engine power while heat-relateddegradation to the downstream components, including a turbochargerturbine and an emission control device, may be decreased compared withoperating each engine cylinder at stoichiometry.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a cylinder that may be included in anengine system.

FIG. 2 shows a schematic depiction of a first example of an enginesystem.

FIG. 3 shows a schematic depiction of a second example of an enginesystem.

FIG. 4 shows a schematic depiction of a third example of an enginesystem.

FIG. 5 depicts an example method for transitioning into and out of astoichiometric mode, a power EGR mode, and a split lambda mode based onengine demand.

FIG. 6 shows a relationship between engine power and EGR rate atdifferent air-fuel ratio splits.

FIG. 7 shows a prophetic example timeline for adjusting engine operationto transition between various operating modes, including astoichiometric mode, a power EGR mode, and a split lambda mode, based onan engine torque demand.

DETAILED DESCRIPTION

The following description relates to systems and methods for increasinga power output of an engine via exhaust gas recirculation (EGR) andpartial engine enrichment. The engine may include various multi-cylinderconfigurations, such as the example engine system configurations shownin FIGS. 2-4, that enable EGR to be selectively drawn from a subset ofthe cylinders for EGR enrichment during the partial engine enrichment.In particular, FIG. 2 shows an inline-4 configuration that includes twocylinders coupled to a first exhaust manifold and two cylinders coupledto a second exhaust manifold, the second exhaust manifold coupled to anEGR passage. FIG. 3 shows a V-6 configuration, where EGR is providedfrom only one of the two cylinder banks. FIG. 4 shows an inline-3configuration that includes the EGR passage coupled to an exhaust runnerof one cylinder. Further, each cylinder of the engine may have acylinder configuration, such as shown in FIG. 1. A controller maytransition the engine into and out of operating with the partialenrichment, referred to herein as a split lambda mode, via the examplemethod of FIG. 5. For example, transitioning into the split lambda modemay include a series of EGR rate, air-fuel ratio, and spark timingadjustments. As another example, engine power may be increased via EGRalone prior to the EGR rate reaching a threshold while operating allcylinders of the engine at stoichiometry, referred to herein as a powerEGR mode. Further, the engine may be operated in a stoichiometric modewithout using EGR to increase the engine power when the engine demand islower (e.g., lower than a threshold). FIG. 6 illustrates a relationshipbetween EGR rate and engine power at different lambda splits (e.g., adifference between a rich air-fuel ratio of a first cylinder group and alean air-fuel ratio of a second cylinder group) and generallyillustrates how the EGR rate and the lambda split are adjusted whiletransitioning between the power EGR mode and the split lambda mode. Anexample timeline illustrating transitioning between the stoichiometricmode, the power EGR mode, and the split lambda mode based on an enginetorque demand is shown in FIG. 7. In this way, the engine power outputmay be increased without increasing vehicle emissions or degradingexhaust components via temperature increases.

Turning now to the figures, FIG. 1 shows a partial view of a singlecylinder 130 of an internal combustion engine 10 that may be included ina vehicle 5. Internal combustion engine 10 may be a multi-cylinderengine, and different engine system configurations for engine 10 will bedescribed below with respect to FIGS. 2-4. Cylinder (e.g., combustionchamber) 130 includes a coolant sleeve 114 and cylinder walls 132, witha piston 136 positioned therein and connected to a crankshaft 140.Combustion chamber 130 is shown communicating with an intake manifold 44via an intake valve 4 and an intake port 22 and with an exhaust port 86via exhaust valve 8.

In the depicted view, intake valve 4 and exhaust valve 8 are located atan upper region of combustion chamber 130. Intake valve 4 and exhaustvalve 8 may be controlled by a controller 12 using respective camactuation systems including one or more cams. The cam actuation systemsmay utilize one or more of cam profile switching (CPS), variable camtiming (VCT), variable valve timing (VVT), and/or variable valve lift(VVL) systems to vary valve operation. In the depicted example, intakevalve 4 is controlled by an intake cam 151, and exhaust valve 8 iscontrolled by an exhaust cam 153. The intake cam 151 may be actuated viaan intake valve timing actuator 101 and the exhaust cam 153 may beactuated via an exhaust valve timing actuator 103 according to setintake and exhaust valve timings, respectively. In some examples, theintake valves and exhaust valves may be deactivated via the intake valvetiming actuator 101 and exhaust valve timing actuator 103, respectively.For example, the controller may send a signal to the exhaust valvetiming actuator 103 to deactivate exhaust valve 8 such that it remainsclosed and does not open at its set timing. The position of intake cam151 and exhaust cam 153 may be determined by camshaft position sensors155 and 157, respectively.

In some examples, the intake and/or exhaust valve may be controlled byelectric valve actuation. For example, cylinder 130 may alternativelyinclude an intake valve controlled via electric valve actuation and anexhaust valve controlled via cam actuation, including CPS and/or VCTsystems. In still other examples, the intake and exhaust valves may becontrolled by a common valve actuator or actuation system or a variablevalve timing actuator or actuation system.

Cylinder 130 can have a compression ratio, which is a ratio of volumeswhen piston 136 is at bottom dead center to top dead center.Conventionally, the compression ratio is in a range of 9:1 to 10:1.However, in some examples where different fuels are used, thecompression ratio may be increased. This may happen, for example, whenhigher octane fuels or fuels with higher latent enthalpy of vaporizationare used. The compression ratio may also be increased if directinjection is used due to its effect on engine knock.

In some examples, each cylinder of engine 10 may include a spark plug 92for initiating combustion. An ignition system 88 can provide an ignitionspark to combustion chamber 130 via spark plug 92 in response to a sparkadvance signal SA from controller 12, under select operating modes.However, in some examples, spark plug 92 may be omitted, such as whereengine 10 initiates combustion by auto-ignition or by injection of fuel,such as when engine 10 is a diesel engine.

As a non-limiting example, cylinder 130 is shown including one fuelinjector 66. Fuel injector 66 is shown coupled directly to combustionchamber 130 for injecting fuel directly therein in proportion to apulse-width of a signal FPW received from controller 12 via anelectronic driver 168. In this manner, fuel injector 66 provides what isknown as direct injection (hereafter also referred to as “DI”) of fuelinto cylinder 130. While FIG. 1 shows injector 66 as a side injector, itmay also be located overhead of the piston, such as near the position ofspark plug 92. Such a position may increase mixing and combustion whenoperating the engine with an alcohol-based fuel due to the lowervolatility of some alcohol-based fuels. Alternatively, the injector maybe located overhead and near the intake valve to improve mixing. Inanother example, injector 66 may be a port injector providing fuel intothe intake port upstream of cylinder 130.

Fuel may be delivered to fuel injector 66 from a high pressure fuelsystem 180 including one or more fuel tanks, fuel pumps, and a fuelrail. Alternatively, fuel may be delivered by a single stage fuel pumpat a lower pressure. Further, while not shown, the fuel tanks mayinclude a pressure transducer providing a signal to controller 12. Fueltanks in fuel system 180 may hold fuel with different fuel qualities,such as different fuel compositions. These differences may includedifferent alcohol content, different octane, different heats ofvaporization, different fuel blends, and/or combinations thereof, etc.In some examples, fuel system 180 may be coupled to a fuel vaporrecovery system including a canister for storing refueling and diurnalfuel vapors. The fuel vapors may be purged from the canister to theengine cylinders during engine operation when purge conditions are met.

Engine 10 may be controlled at least partially by controller 12 and byinput from a vehicle operator 113 via an accelerator pedal 116 and anaccelerator pedal position sensor 118 and via a brake pedal 117 and abrake pedal position sensor 119. The accelerator pedal position sensor118 may send a pedal position signal (PP) to controller 12 correspondingto a position of accelerator pedal 116, and the brake pedal positionsensor 119 may send a brake pedal position (BPP) signal to controller 12corresponding to a position of brake pedal 117. Controller 12 is shownin FIG. 1 as a microcomputer, including a microprocessor unit 102,input/output ports 104, an electronic storage medium for executableprograms and calibration values shown as a read only memory 106 in thisparticular example, random access memory 108, keep alive memory 110, anda data bus. Storage medium read-only memory 106 can be programmed withcomputer readable data representing instructions executable bymicroprocessor 102 for performing the methods and routines describedherein as well as other variants that are anticipated but notspecifically listed. Controller 12 may receive various signals fromsensors coupled to engine 10, in addition to those signals previouslydiscussed, including a measurement of inducted mass air flow (MAF) frommass air flow sensor 48, an engine coolant temperature signal (ECT) froma temperature sensor 112 coupled to coolant sleeve 114, a profileignition pickup signal (PIP) from a Hall effect sensor 120 (or othertype) coupled to crankshaft 140, a throttle position (TP) from athrottle position sensor coupled to a throttle 62, and an absolutemanifold pressure signal (MAP) from a MAP sensor 122 coupled to intakemanifold 44. An engine speed signal, RPM, may be generated by controller12 from signal PIP. The manifold pressure signal MAP from the manifoldpressure sensor may be used to provide an indication of vacuum orpressure in the intake manifold.

Based on input from one or more of the above-mentioned sensors,controller 12 may adjust one or more actuators, such as fuel injector66, throttle 62, spark plug 92, the intake/exhaust valves and cams, etc.The controller may receive input data from the various sensors, processthe input data, and trigger the actuators in response to the processedinput data based on instruction or code programmed therein correspondingto one or more routines, an example of which is described with respectto FIG. 5.

In some examples, vehicle 5 may be a hybrid vehicle with multiplesources of torque available to one or more vehicle wheels 160. In otherexamples, vehicle 5 is a conventional vehicle with only an engine. Inthe example shown in FIG. 1, the vehicle includes engine 10 and anelectric machine 161. Electric machine 161 may be a motor or amotor/generator and thus may also be referred to herein as an electricmotor. Electric machine 161 receives electrical power from a tractionbattery 170 to provide torque to vehicle wheels 160. Electric machine161 may also be operated as a generator to provide electrical power tocharge battery 170, for example, during a braking operation.

Crankshaft 140 of engine 10 and electric machine 161 are connected via atransmission 167 to vehicle wheels 160 when one or more clutches 166 areengaged. In the depicted example, a first clutch 166 is provided betweencrankshaft 140 and electric machine 161, and a second clutch 166 isprovided between electric machine 161 and transmission 167. Controller12 may send a signal to an actuator of each clutch 166 to engage ordisengage the clutch, so as to connect or disconnect crankshaft 140 fromelectric machine 161 and the components connected thereto, and/orconnect or disconnect electric machine 161 from transmission 167 and thecomponents connected thereto. Transmission 167 may be a gearbox, aplanetary gear system, or another type of transmission. The powertrainmay be configured in various manners including as a parallel, a series,or a series-parallel hybrid vehicle.

As mentioned above, FIG. 1 shows only one cylinder of multi-cylinderengine 10. Referring now to FIG. 2, a schematic diagram of a firstexample engine system 200 is shown, which may be included in thepropulsion system of vehicle 5 of FIG. 1. For example, engine system 200provides a first example engine configuration of engine 10 introduced inFIG. 1. As such, components previously introduced in FIG. 1 arerepresented with the same reference numbers and are not re-introduced.In the example shown in FIG. 2, engine 10 includes cylinders 13, 14, 15,and 18, arranged in an inline-4 configuration, although otherconfigurations of engine 10 will be described with respect to FIGS. 3and 4. The engine cylinders may be capped on the top by a cylinder head.With respect to FIG. 2, cylinders 14 and 15 are referred to herein asthe inner (or inside) cylinders, and cylinders 13 and 18 are referred toherein as the outer (or outside) cylinders. The cylinders shown in FIG.2 may each have a cylinder configuration, such as the cylinderconfiguration described above with respect to FIG. 1.

Each of cylinders 13, 14, 15, and 18 includes at least one intake valve4 and at least one exhaust valve 8. The intake and exhaust valves may bereferred to herein as cylinder intake valves and cylinder exhaustvalves, respectively. As explained above with reference to FIG. 1, atiming (e.g., opening timing, closing timing, opening duration, etc.) ofeach intake valve 4 and each exhaust valve 8 may be controlled viavarious valve timing systems.

Each cylinder receives intake air (or a mixture of intake air andrecirculated exhaust gas, as will be elaborated below) from intakemanifold 44 via an air intake passage 28. Intake manifold 44 is coupledto the cylinders via intake ports (e.g., runners) 22. In this way, eachcylinder intake port can selectively communicate with the cylinder it iscoupled to via a corresponding intake valve 4. Each intake port maysupply air, recirculated exhaust gas, and/or fuel to the cylinder it iscoupled to for combustion.

As described above with respect to FIG. 1, a high pressure fuel systemmay be used to generate fuel pressures at the fuel injector 66 coupledto each cylinder. For example, controller 12 may inject fuel into eachcylinder at a different timing such that fuel is delivered to eachcylinder at an appropriate time in an engine cycle. As used herein,“engine cycle” refers to a period during which each engine cylinderfires once in a designated cylinder firing order. A distributorlessignition system may provide an ignition spark to cylinders 13, 14, 15,and 18 via the corresponding spark plug 92 in response to the signal SAfrom controller 12 to initiate combustion. A timing of the ignitionspark may be individually optimized for each cylinder, as will befurther described below with respect to FIG. 5.

Inside cylinders 14 and 15 are each coupled to one exhaust port (e.g.,runner) 86 and outside cylinders 13 and 18 are each coupled to anexhaust port 87 for channeling combustion exhaust gases to an exhaustsystem 84. Each exhaust port 86 and 87 can selectively communicate withthe cylinder it is coupled to via the corresponding exhaust valve 8.Specifically, as shown in FIG. 2, cylinders 14 and 15 channel exhaustgases to a first exhaust manifold 81 via exhaust ports 86, and cylinders13 and 18 channel exhaust gases to a second exhaust manifold 85 viaexhaust ports 87. First exhaust manifold 81 and second exhaust manifold85 do not directly communicate with one another (e.g., no passagedirectly couples the two exhaust manifolds to one another).

Engine system 200 further includes a turbocharger 164, including aturbine 165 and an intake compressor 162 coupled on a common shaft (notshown). In the example shown in FIG. 2, turbine 165 is a twin scroll (ordual volute) turbine. In such an example, a first, hotter scroll of thetwin scroll turbine may be coupled to second exhaust manifold 85, and asecond, cooler scroll of the twin scroll turbine may be coupled to firstexhaust manifold 81 such that first exhaust manifold 81 and secondexhaust manifold 85 remain separated up to the turbine wheel. Forexample, the two scrolls may each introduce gas around the entireperimeter of the wheel, but at different axial locations. Alternatively,the two scrolls may each introduce gas to the turbine over a portion ofthe perimeter, such as over approximately 180 degrees of the perimeter.In another example, engine 10 may include a monoscroll turbine. In someexamples of the monoscroll turbine, first exhaust manifold 81 and secondexhaust manifold 85 may combine prior to reaching the turbine wheel. Thetwin scroll configuration may provide greater power to the turbine wheelcompared with the monoscroll configuration by providing a minimum volume(e.g., exhaust gas from two cylinders and a smaller manifold volume)from a given combustion event. In contrast, the monoscroll configurationenables use of lower cost turbines that have higher temperaturetolerances.

Rotation of turbine 165 drives rotation of compressor 162, disposedwithin intake passage 28. As such, the intake air becomes boosted (e.g.,pressurized) at the compressor 162 and travels downstream to intakemanifold 44. Exhaust gases exit turbine 165 into an exhaust passage 74.In some examples, a wastegate may be coupled across turbine 165 (notshown). Specifically, a wastegate valve may be included in a bypasscoupled between an inlet of turbine 165 and exhaust passage 74,downstream of an outlet of turbine 165. The wastegate valve may controlan amount of exhaust gas flowing through the bypass and to the outlet ofturbine. For example, as an opening of the wastegate valve increases, anamount of exhaust gas flowing through the bypass and not through turbine165 may increase, thereby decreasing an amount of power available fordriving turbine 165 and compressor 162. As another example, as theopening of the wastegate valve decreases, the amount of exhaust gasflowing through the bypass decreases, thereby increasing the amount ofpower available for driving turbine 165 and compressor 162. In this way,a position of the wastegate valve may control an amount of boostprovided by turbocharger 164. In other examples, turbine 165 may be avariable geometry turbine (VGT) including adjustable vanes to change aneffective aspect ratio of turbine 165 as engine operating conditionschange to provide a desired boost pressure. Thus, increasing the speedof turbocharger 164, such as by further closing the wastegate valve oradjusting turbine vanes, may increase the amount of boost provided, anddecreasing the speed of turbocharger 164, such as by further opening thewastegate valve or adjusting the turbine vanes, may decrease the amountof boost provided.

After exiting turbine 165, exhaust gases flow downstream in exhaustpassage 74 to an emission control device 70. Emission control device 70may include one or more emission control devices, such as one or morecatalyst bricks and/or one or more particulate filters. For example,emission control device may 70 include a three-way catalyst configuredto chemically reduce nitrogen oxides (NOx) and oxidize carbon monoxide(CO) and hydrocarbons (HC). In some examples, emission control device 70may additionally or alternatively include a gasoline particulate filter(GPF). After passing through emission control device 70, exhaust gasesmay be directed out to a tailpipe. As an example, the three-way catalystmay be maximally effective at treating exhaust gas with a stoichiometricair-fuel ratio (AFR), as will be elaborated below.

Exhaust passage 74 further includes a plurality of exhaust sensors inelectronic communication with controller 12, which is included in acontrol system 17. As shown in FIG. 2, exhaust passage 74 includes afirst oxygen sensor 90 positioned upstream of emission control device70. First oxygen sensor 90 may be configured to measure an oxygencontent of exhaust gas entering emission control device 70. Exhaustpassage 74 may include one or more additional oxygen sensors positionedalong exhaust passage 74, such as a second oxygen sensor 91 positioneddownstream of emission control device 70. As such, second oxygen sensor91 may be configured to measure the oxygen content of the exhaust gasexiting emission control device 70. In one example, one or more ofoxygen sensor 90 and oxygen sensor 91 may be universal exhaust gasoxygen (UEGO) sensors. Alternatively, a two-state exhaust gas oxygensensor may be substituted for at least one of oxygen sensors 90 and 91.Exhaust passage 74 may include various other sensors, such as one ormore temperature and/or pressure sensors. For example, as shown in FIG.2, a sensor 96 is positioned within exhaust passage 74 upstream ofemission control device 70. Sensor 96 may be a pressure and/ortemperature sensor. As such, sensor 96 may be configured to measure thepressure and/or temperature of exhaust gas entering emission controldevice 70.

Second exhaust manifold 85 is directly coupled to an exhaust gasrecirculation (EGR) passage 50 included in an EGR system 56. EGR passage50 is coupled between second exhaust manifold 85 and intake passage 28,downstream of compressor 162. As such, exhaust gases are directed fromsecond exhaust manifold 85 (and not first exhaust manifold 81) to airintake passage 28, downstream of compressor 162, via EGR passage 50,which provides high-pressure EGR. However, in other examples, EGRpassage 50 may be coupled to intake passage 28 upstream of compressor162.

As shown in FIG. 2, EGR passage 50 may include an EGR cooler 52configured to cool exhaust gases flowing from second exhaust manifold 85to intake passage 28 and may further include an EGR valve 54 disposedtherein. Controller 12 is configured to actuate and adjust a position ofEGR valve 54 in order to control a flow rate and/or amount of exhaustgases flowing through EGR passage 50. When EGR valve 54 is in a closed(e.g., fully closed) position, no exhaust gases may flow from secondexhaust manifold 85 to intake passage 28. When EGR valve 54 is in anopen position (e.g., from partially open to fully open), exhaust gasesmay flow from second exhaust manifold 85 to intake passage 28.Controller 12 may adjust the EGR valve 54 into a plurality of positionsbetween fully open and fully closed. In other examples, controller 12may only adjust EGR valve 54 to be either fully open or fully closed.Further, in some examples, a pressure sensor 34 may be arranged in EGRpassage 50 upstream of EGR valve 54.

As shown in FIG. 2, EGR passage 50 is coupled to intake passage 28downstream of a charge air cooler (CAC) 40. CAC 40 is configured to coolintake air as it passes through CAC 40. In an alternative example, EGRpassage 50 may be coupled to intake passage 28 upstream of CAC 40 (anddownstream of compressor 162). In some such examples, EGR cooler 52 maynot be included in EGR passage 50, as CAC cooler 40 may cool both theintake air and recirculated exhaust gases. EGR passage 50 may furtherinclude an oxygen sensor 36 disposed therein and configured to measurean oxygen content of exhaust gases flowing through EGR passage 50 fromsecond exhaust manifold 85. In some examples, EGR passage 50 may includeadditional sensors, such as temperature and/or humidity sensors, todetermine a composition and/or quality of the exhaust gas beingrecirculated to intake passage 28 from second exhaust manifold 85.

Intake passage 28 further includes throttle 62. As shown in FIG. 2,throttle 62 is positioned downstream of CAC 40 and downstream of whereEGR passage 50 couples to intake passage 28 (e.g., downstream of ajunction between EGR passage 50 and intake passage 28). A position of athrottle plate 64 of throttle 62 may be adjusted by controller 12 via athrottle actuator (not shown) communicatively coupled to controller 12.By modulating throttle 62 while operating compressor 162, a desiredamount of fresh air and/or recirculated exhaust gas may be delivered tothe engine cylinders at a boosted pressure via intake manifold 44.

To reduce compressor surge, at least a portion of the air chargecompressed by compressor 162 may be recirculated to the compressorinlet. A compressor recirculation passage 41 may be provided forrecirculating compressed air from a compressor outlet, upstream of CAC40, to a compressor inlet. A compressor recirculation valve (CRV) 42 maybe provided for adjusting an amount of flow recirculated to thecompressor inlet. In one example, CRV 42 may be actuated open via acommand from controller 12 in response to actual or expected compressorsurge conditions.

Intake passage 28 may include one or more additional sensors (such asadditional pressure, temperature, flow rate, and/or oxygen sensors). Forexample, as shown in FIG. 2, intake passage 28 includes MAF sensor 48disposed upstream of compressor 162 in intake passage 28. An intakepressure and/or temperature sensor 31 is also positioned in intakepassage 28 upstream of compressor 162. An intake oxygen sensor 35 may belocated in intake passage 28 downstream of compressor 162 and upstreamof CAC 40. An additional intake pressure sensor 37 may be positioned inintake passage 28 downstream of CAC 40 and upstream of throttle 62(e.g., a throttle inlet pressure sensor). In some examples, as shown inFIG. 2, an additional intake oxygen sensor 39 may be positioned inintake passage 28 between CAC 40 and throttle 62, downstream of thejunction between EGR passage 50 and intake passage 28. Further, MAPsensor 122 and an intake manifold temperature sensor 123 are shownpositioned within intake manifold 44, upstream of the engine cylinders.

Engine 10 may be controlled at least partially by control system 17,including controller 12, and by input from the vehicle operator (asdescribed above with respect to FIG. 1). Control system 17 is shownreceiving information from a plurality of sensors 16 (various examplesof which are described herein) and sending control signals to aplurality of actuators 83. As one example, sensors 16 may include thepressure, temperature, and oxygen sensors located within intake passage28, intake manifold 44, exhaust passage 74, and EGR passage 50, asdescribed above. Other sensors may include a throttle inlet temperaturesensor for estimating a throttle air temperature (TCT) coupled upstreamof throttle 62 in the intake passage. Further, it should be noted thatengine 10 may include all or only a portion of the sensors shown in FIG.2. As another example, actuators 83 may include fuel injectors 66,throttle 62, CRV 42, EGR valve 54, and spark plugs 92. Actuators 83 mayfurther include various camshaft timing actuators coupled to thecylinder intake and exhaust valves (as described above with reference toFIG. 1). Controller 12 may receive input data from the various sensors,process the input data, and trigger the actuators in response to theprocessed input data based on instruction or code programmed in a memoryof controller 12 corresponding to one or more routines. An examplecontrol routine (e.g., method) is described herein at FIG. 5.

The configuration of engine system 200 may enable engine performanceenhancement while reducing vehicle emissions. In particular, byincluding separate exhaust manifolds that do not directly communicateand that receive exhaust gases from different cylinders, the gasesreceived by first exhaust manifold 81 may have a different AFR than thegases received by second exhaust manifold 85. Herein, the AFR will bediscussed as a relative AFR, defined as a ratio of an actual AFR of agiven mixture to stoichiometry and represented by lambda (λ). A lambdavalue of 1 occurs during stoichiometric operation (e.g., atstoichiometry), wherein the air-fuel mixture produces a completecombustion reaction. A rich feed (λ<1) results from air-fuel mixtureswith more fuel relative to stoichiometry. For example, when a cylinderis enriched, more fuel is supplied to the cylinder via fuel injector 66than for producing a complete combustion reaction with an amount of airin the cylinder, resulting in excess, unreacted fuel. In contrast, alean feed (λ>1) results from air-fuel mixtures with less fuel relativeto stoichiometry. For example, when a cylinder is enleaned, less fuel isdelivered to the cylinder via fuel injector 66 than for producing acomplete combustion reaction with the amount of air in the cylinder,resulting in excess, unreacted air. During nominal engine operation, theAFR may fluctuate about stoichiometry, such as by k generally remainingwithin 2% of stoichiometry. For example, the engine may transition fromrich to lean and from lean to rich between injection cycles, resultingin an “average” operation at stoichiometry.

Further, during some engine operating conditions, the AFR may bedeviated from stoichiometry. As one example, global enrichment (in whicheach cylinder is operated with a rich AFR) is a conventional performanceenhancement strategy to increase engine power. Generally, highercylinder air charges result in more engine torque and thus more enginepower, with the cylinder fueling correspondingly increased based on thehigher air charge to maintain the enrichment. In particular, theadditional, unreacted fuel cools engine system components, including thedownstream turbine 165 and emission control device 70, enabling more airflow for increased power while reducing heat-related degradation to thedownstream components (versus operating at stoichiometry with the highercylinder air charge). However, as mentioned above, emission controldevice 70 is most effective at stoichiometry, and thus, the abovedescribed global enrichment strategy results in increased vehicleemissions, particularly increased CO and HC emissions.

Therefore, according to the present disclosure, such as when high enginetorque (or high engine power) is demanded, a first set of cylinders maybe operated at a first, rich AFR, and a second, remaining set of theengine cylinders may be operated at a second, lean AFR. Such operationwill be referred to herein as “split lambda” operation (or operation ina split lambda mode). In particular, the inside cylinders may beoperated at the lean AFR, resulting cylinders 14 and 15 feeding leanexhaust gas to first exhaust manifold 81, and the outside cylinders maybe operated at the rich AFR, resulting in cylinders 13 and 14 feedingrich exhaust gas to second exhaust manifold 85. The lean exhaust gas infirst exhaust manifold 81 may be isolated from the rich exhaust gas insecond exhaust manifold 85 prior to mixing at and downstream of turbine165. Further, a degree of enleanment of the second set of cylinders mayselected based on a degree of enrichment of the first set of cylindersso that the exhaust gas from the first set of cylinders may mix with theexhaust gas from the second set of cylinders to form a stoichiometricmixture, even while none of the cylinders are operated at stoichiometry.Further still, the degree of enrichment of the first set of cylinders(and the degree of enleanment of the second set of cylinders) is greaterthan the typical fluctuation about stoichiometry performed duringnominal engine operation. As an example, the first set of cylinders maybe operated at a rich AFR having a lambda value in a range from 0.95-0.8(e.g., 5-20% rich).

By maintaining engine 10 at overall (e.g., global) stoichiometry, evenwhile operating in the split lambda operating mode, tailpipe emissionsmay be reduced. For example, operating in the split lambda mode mayresult in a substantial reduction in CO emissions compared toconventional enriched engine operation (e.g., a 90% reduction) whileproviding increased engine cooling and increased engine power, similarto the conventional enriched engine operation. As an example, controller12 may transition engine 10 to and from the split lambda operating moderesponsive to an increased engine demand, as will be further describedwith respect to FIG. 5.

Further, because EGR passage 50 is coupled to second exhaust manifold85, which receives the enriched exhaust gas from outside cylinders 13and 18 in during the split lambda operation, the exhaust gasrecirculated to intake passage 28 (and supplied to every cylinder ofengine 10) may be enriched. The enriched EGR contains relatively highconcentrations (or amounts) of CO and hydrogen gas compared with leanEGR and stoichiometric EGR. CO and hydrogen gas have high effectiveoctane numbers, offsetting the knock limit of each cylinder and creatingan opportunity for additional spark advance to both the enriched andenleaned cylinders. The spark advance provides additional temperaturerelief to turbine 165 and emission control device 70, enabling even moreair flow (and thus engine power) than when engine 10 is operated withoutenriched EGR. Thus, the cooled, enriched EGR may provide additionalknock and efficiency benefits to engine 10. Further still, even prior tooperating in the split lambda mode and enriching the EGR, providing EGRat high engine loads may provide engine cooling, enabling engine airflow to be increased relative to when no EGR is provided. Such operationwill be referred to herein as a power EGR mode and will be furtherdescribed with respect to FIG. 5.

Other engine system configurations may also enable operation in thesplit lambda mode with enriched EGR for increased engine power andreduced emissions. Next, FIG. 3 shows a second example configuration ofengine 10. Specifically, FIG. 3 shows an example engine system 300 withengine 10 including cylinders 13, 14, 15, 19, 20, and 21 in a V-6configuration. However, other numbers of engine cylinders are alsopossible, such as a V-8 configuration. Except for the differencesdescribed below, engine system 300 may be substantially identical toengine system 200 of FIG. 2. As such, components previously introducedin FIGS. 1 and 2 are represented with the same reference numbers and arenot re-introduced.

In the example of engine system 300, engine 10 includes two enginebanks, first engine bank 312 and second engine bank 314. Specifically,first engine bank 312 includes cylinders 13, 14, and 15, each coupled tointake manifold 44 via intake ports 22, and second engine bank 314includes cylinders 19, 20, and 21, each coupled to intake manifold 44via intake ports 22. Each of cylinders 13, 14, and 15 of first enginebank 312 exhausts combustion gases to first exhaust manifold 81 viaexhaust ports 86. From first exhaust manifold 81, the gases may bedirected to a turbine 175 of a turbocharger 174. In contrast, each ofcylinders 19, 20, and 21 of second engine bank 314 exhausts combustiongases to second exhaust manifold 85, which is separate from exhaustmanifold 85, via exhaust ports 87. For example, no passages directlycouple first exhaust manifold 81 and second exhaust manifold 85. Fromsecond exhaust manifold 85, the gases may be directed to turbine 165 ofturbocharger 164, which is different than turbocharger 174. For example,turbine 175 is positioned in a first exhaust passage 77 and receivesexhaust gases exclusively from first exhaust manifold 81 for driving acompressor 172 positioned in an intake passage 29. Turbine 165 ispositioned in a second exhaust passage 76 and receives exhaust gasesexclusively from exhaust manifold 85 for driving compressor 162positioned in intake passage 28. For example, as shown, compressor 172may be coupled in parallel with compressor 162.

Thus, in the example configuration of engine system 300, exhaust system84 includes two separate exhaust manifolds, first exhaust manifold 81and second exhaust manifold 85, each coupled to engine cylinders of asingle engine bank. Further, exhaust system 84 includes twoturbochargers, turbocharger 164 and turbocharger 174, each having aturbine positioned to receive exhaust gas from only one of the twoexhaust manifolds.

First exhaust passage 77 and second exhaust passage 76 merge and arecoupled to exhaust passage 74 downstream of turbines 175 and 165,respectively. Exhaust passage 74 serves as a common exhaust passage. Insome examples, one or both of exhaust passages 77 and 76 may include aclose-coupled catalyst downstream of the corresponding turbine andupstream of exhaust passage 74. In the example shown in FIG. 3, a firstclose-coupled catalyst 78 is positioned in first exhaust passage 77downstream of turbine 175 and upstream of where first exhaust passage 77couples with common exhaust passage 74, and a second close-coupledcatalyst 72 is positioned in second exhaust passage 76 downstream ofturbine 165 and upstream of where second exhaust passage 76 couples withcommon exhaust passage 74. In contrast, emission control device 70 ispositioned in common exhaust passage 74. Thus, while first close-coupledcatalyst 78 receives exhaust gas exclusively from first engine bank 312(e.g., via first exhaust manifold 81 and turbine 175) and secondclose-coupled catalyst 72 receives exhaust gas exclusively from secondengine bank 314 (e.g., via second exhaust manifold 85 and turbine 165),emission control device 70 receives exhaust gas from both first enginebank 312 and second engine bank 314, and all of the exhaust gas directedout the tailpipe passes through exhaust passage 74 and emission controldevice 70. However, in other examples, first close-coupled catalyst 78and second close-coupled catalyst 72 may be omitted.

When first close-coupled catalyst 78 and second close-coupled catalyst72 are included, they may reduce vehicle emissions prior to operating inthe split lambda mode (e.g., during an engine cold start). For example,due to being positioned closer to engine 10, first close-coupledcatalyst 78 and second close-coupled catalyst 72 may receive more heatfrom the engine than emission control device 70 and may thereforeachieve light-off faster. However, first close-coupled catalyst 78 andsecond close-coupled catalyst 72 may be less efficient while operatingin the split lambda mode due to receiving only rich or lean exhaust gas.In such examples, emission control device 70 may effectively treatexhaust gas components not treated by first close-coupled catalyst 78and second close-coupled catalyst 72.

As shown in FIG. 3, exhaust passage 74 includes first oxygen sensor 90and sensor 96, each positioned upstream of emission control device 70,and the optional second oxygen sensor 91, positioned downstream ofemission control device 70, as in engine system 200 described above withrespect to FIG. 2. In other examples, additionally or alternatively,exhaust gas sensors, such as oxygen, temperature and/or pressuresensors, may be coupled to first exhaust passage 77 and/or secondexhaust passage 76. For example, an oxygen sensor may be coupled tofirst exhaust passage 77 upstream of first close-coupled catalyst 78and/or coupled to second exhaust passage 76 upstream of secondclose-coupled catalyst 72.

Intake passages 28 and 29 may be configured as two parallel intakepassages that merge and couple to a common intake passage 30 upstream ofthrottle 62. As shown in FIG. 3, intake passage 28 includes CAC 40, asintroduced in FIG. 2, and intake passage 29 includes a second CAC 43.However, in other examples, a single charge air cooler may be included,such as positioned in common intake passage 30 upstream of throttle 62.Intake passage 29 may include a second set of some or all of the varioussensors positioned in intake passage 28 and described above with respectto FIG. 2 for determining various qualities of the intake air beingprovided engine 10. For example, intake passage 29 is shown including aMAF sensor 49, a temperature sensor 32, and an intake oxygen sensor 33.Alternatively, only one of intake passages 28 and 29 may include eachsensor. For example, intake passage 28 may include MAF sensor 48 andtemperature sensor 31 (and not intake oxygen sensor 35), and intakepassage 29 may include intake oxygen sensor 33 (and not MAF sensor 49and temperature sensor 32). As another example, intake passage 29 mayinclude MAF sensor 49 (and not temperature sensor 32 and intake oxygensensor 33), and intake passage 28 may include temperature sensor 31 andintake oxygen sensor 35 (and not MAF sensor 48).

Further, intake passage 29 may include a compressor recirculationpassage 46 for recirculating compressed air from an outlet of compressor172, upstream of CAC 43, to an inlet of compressor 172. A CRV 45 may beprovided for adjusting an amount of flow recirculated to the inlet ofcompressor 172. Thus, compressor recirculation passage 46 and CRV 45 mayfunction similarly to compressor recirculation passage 41 and CRV 42,respectively, as described above with respect to FIG. 2.

In the example of engine system 300, EGR passage 50 is directly coupledto second exhaust manifold 85 and is not coupled to first exhaustmanifold 81. Thus, EGR system 56 recirculates exhaust gases produced bycombustion in second engine bank 314 and not first engine bank 312 whenEGR valve 54 is at least partially open. Further EGR passage 50 is showncoupled to intake passage 28 downstream of CAC 40 and upstream of whereintake passage 28 couples to common intake passage 30. However, in otherexamples, EGR passage 50 may be coupled to common intake passage 30,such as upstream of throttle 62. Because intake passage 28 flows intakeair to common intake passage 30, which provides intake air to everycylinder of engine 10 via intake manifold 44, when EGR is requested, therecirculated exhaust gas may be provided to each cylinder of engine 10.

Due to the configuration of EGR system 56, the cylinders of secondengine bank 314 may be operated at the first, rich AFR, and thecylinders of first engine bank 312 may be operated at the second, leanAFR. In particular, cylinders 19, 30, and 21 may be operated at the richAFR, resulting in rich exhaust gas flowing to second exhaust manifold85, a portion of which may be recirculated to intake passage 28 via EGRpassage 50. Cylinders 13, 14, and 15 may be operated at the lean AFR,resulting in lean exhaust gas flowing to first exhaust manifold 81. Thelean exhaust gas in first exhaust manifold 81 is isolated from the richexhaust gas in second exhaust manifold 85 prior to mixing at exhaustpassage 74. Thus, while rich exhaust gas may flow through secondclose-coupled catalyst 72 and lean exhaust gas may flow through firstclose-coupled catalyst 78 during the split lambda operation, the exhaustgas flowing through emission control device 70 may be maintainedstoichiometry, on average, to decrease emissions.

Still other engine systems may be operated in the split lambda mode.Turning to FIG. 4, a third example configuration of engine 10 is shown.Specifically, FIG. 4 shows an example engine system 400, with engine 10having an inline-3 configuration instead of the inline-4 configurationof engine system 200 of FIG. 2. Except for the differences describedbelow, engine system 400 may be substantially identical to engine system200 of FIG. 2. As such, components previously introduced in FIGS. 1-3are represented with the same reference numbers and are notre-introduced.

As mentioned above, in the example of engine system 400, engine 10includes cylinders 13, 14, and 15, arranged in an inline-3configuration. Further, exhaust system 84 of engine system 300 includesonly exhaust manifold 85. As such, exhaust manifold 85 is coupled toeach of cylinders 13, 14, and 15 (e.g., every cylinder of engine 10) viaexhaust ports 87, and exhaust manifold 85 receives exhaust gasesexpelled from all of the cylinders of engine 10. The exhaust gasesreceived by exhaust manifold 85 may be channeled to turbine 165, asdescribed above.

When EGR is provided via EGR system 56, such as when EGR valve 54 is atleast partially open, a portion of the exhaust gas may flow through EGRpassage 50. In the example of engine system 300, EGR passage 50 mayreceive exhaust gas originating from each of cylinders 13, 14, and 15.However, EGR passage 50 is coupled to exhaust port 87 of cylinder 13,upstream of where exhaust port 87 of cylinder 13 joins exhaust manifold85. Due to the position of EGR passage 50 of exhaust port 87 and fluiddynamics within exhaust manifold 85, a much higher proportion of theexhaust gas recirculated through EGR passage 50 may originate fromcombustion within cylinder 13 compared with cylinders 14 and 15. Forexample, at least 80% of the exhaust gas flowing through EGR passage 50may originate from combustion within cylinder 13.

Due to the odd number of cylinders in engine 10 in engine system 400,operation in the split lambda mode may be different than when the enginehas an even number of cylinders (such as in engine system 200 of FIG. 2and engine system 300 of FIG. 3). For example, cylinders 13, 14, and 15each may be operated with a different AFR while the exhaust gas thatflows from exhaust manifold 85 to emission control device 70 maintainsglobal stoichiometry. That is, a first cylinder may be operated at afirst, rich AFR, a second cylinder may be operated at a second,stoichiometric AFR, and a third, remaining cylinder may be operated at athird, lean AFR, resulting in a stoichiometric mixture upstream ofemission control device 70. Specifically, cylinder 13 may be operated atthe rich AFR, cylinder 14 may be operated at stoichiometry, and cylinder15 may be operated at the lean AFR. In another example, cylinder 14 maybe operated at the lean AFR while cylinder 15 may be operated atstoichiometry. However, cylinder 13 may be selectively enriched so thatthe exhaust gas recirculated via EGR system 56 is enriched in order toachieve the spark advance and additional cooling benefits describedabove with respect to FIG. 2.

Thus, the systems of FIGS. 2-4 provide three example engineconfigurations (e.g., an inline configuration having an even number ofcylinders, a V-configuration, and an inline configuration having an oddnumber of cylinders) and descriptions of how each of the three engineconfigurations enables operation in the split lambda mode with enrichedEGR, thereby increasing engine power while decreasing fuel usage andreducing vehicle emissions. Note that the number of cylinders in eachconfiguration may be changed without parting from the scope of thisdisclosure.

Next, FIG. 5 provides an example method 500 for adjusting engineoperation based on engine demand, including transitioning into and outof the split lambda operating mode. For example, the split lambdacombustion strategy inherently causes an imbalance between rich and leancylinders due to different burn rates, which may result in enginevibrations. Therefore, method 500 provides a control strategy formitigating this imbalance in order to reduce the engine vibrations.Method 500 additionally includes a control strategy for transitioninginto and out of a power EGR mode, during which cooled EGR is provided toincrease engine output via exhaust component temperature relief. Forexample, the power EGR mode may be used to produce engine power outputsthat are greater than can be provided via boost without EGR due toexhaust temperature limitations and less than can be provided via thesplit lambda mode. Instructions for carrying out method 500 and the restof the methods included herein may be executed by a controller (e.g.,controller 12 of FIGS. 1-4) based on instructions stored on a memory ofthe controller and in conjunction with signals received from sensors ofthe engine system, such as the sensors described above with reference toFIGS. 1-4. The controller may employ engine actuators of the enginesystem (e.g., fuel injector 66 of FIGS. 1-4, spark plug 92 of FIGS. 1-4,and EGR valve 54 of FIGS. 2-4) to adjust engine operation according tothe methods described below.

At 502, method 500 includes estimating and/or measuring engine operatingconditions. The operating conditions may include, for example, a brakepedal position, an accelerator pedal position, ambient temperature andhumidity, barometric pressure, engine speed, engine load, engine torque,engine temperature, mass air flow (MAF), intake manifold pressure (MAP),a commanded AFR, an actual AFR of exhaust gas entering an emissioncontrol device (e.g., emission control device 70 of FIGS. 2-4), anexhaust temperature, etc. As an example, the controller may use theaccelerator pedal position to determine the engine torque demanded by avehicle operator. For example, the controller may input the acceleratorpedal position and the engine speed into an engine map to determine theengine torque demand. Further, the controller may determine engine powerproduced based on the engine torque and the engine speed, such as bymultiplying the engine torque by the engine speed. As another example,the controller may determine a boost pressure provided by a turbocharger(e.g., turbocharger 164 of FIGS. 2-4) based on (e.g., as a function of)MAP and the barometric pressure.

At 504, method 500 includes determining whether the torque demand isgreater than a first threshold torque. The first threshold torque may bea pre-calibrated non-zero engine torque value above which the torquecannot be further increased while operating the engine at stoichiometrywithout risking heat-related degradation to exhaust system components,such as a turbine of the turbocharger (e.g., turbine 165 of FIGS. 2-4)and the emission control device. As mentioned above with respect to FIG.2, more engine air flow (e.g., higher MAF and/or MAP values) results inmore engine power. However, as also mentioned above, this increases thetemperature of the exhaust gas produced, and thus, the temperature ofthe exhaust system components. Therefore, the first threshold torque maybe set based on a threshold exhaust temperature, the threshold exhausttemperature including a pre-calibrated non-zero exhaust temperaturevalue above which exhaust system component degradation may be increased.As an alternative example of the method at 504, it may be determined ifthe engine power demand is greater than a first threshold power, whichmay correspond to the first threshold torque at a given engine speed.

If the torque demand is not greater than the first threshold torque,method 500 proceeds to 542 and includes operating the engine in astoichiometric mode (also referred to herein as a stoichiometricoperating mode). EGR may be provided, with a rate of the EGR adjustedresponsive to engine speed and load, such as to provide EGR duringpart-load operation in order to increase engine efficiency, reduce fuelconsumption, and decrease NOx emissions. Further, an amount of EGRprovided may be limited by combustion stability or flow capability ofthe engine. This is different than providing EGR to increase enginepower, as will be elaborated herein. For example, EGR may be providedwhen the engine load is greater than a first, lower threshold load andless than a second, higher threshold load. The first threshold load maybe a pre-calibrated non-zero load below which EGR may cause unstablecombustion. The second threshold load may be a pre-calibrated non-zeroload above which EGR may reduce engine power. Additionally, boost may beprovided via the turbocharger based on the torque demand. However, boostpressure (e.g., amount of boost) may be limited based on the exhausttemperature, such as to maintain the exhaust temperature below thethreshold exhaust temperature. Thus, the boost pressure may be keptbelow a temperature-limited boost pressure threshold while operating inthe stoichiometric mode. As one example, the temperature-limited boostpressure threshold may correspond to the boost pressure for producingthe first threshold torque.

Method 500 may then end. Further, method 500 may be repeated so that thecontroller may update the operating mode as operating conditions change.For example, the controller may automatically and continuously (e.g., inreal-time) repeat at least parts of method 500 so that changes inoperating conditions, such changes in the torque demand, m be detectedbased on signals received from sensors of the engine system andevaluated to determine if the change in operating conditions warrant achange in the engine operating mode.

Returning to 504, if instead the torque demand is greater than the firstthreshold torque, method 500 proceeds to 506 and includes transitioningthe engine into the power EGR mode. The power EGR mode (also referred toherein as the power EGR operating mode) is different than providing EGRto reduce fuel consumption and NOx emissions during part-load operation,described above. By providing EGR when the torque demand is greater thanthe first threshold torque, combustion temperatures may be decreased,thereby enabling a greater cylinder air charge (e.g., more boost) andcorresponding fueling for increased engine power while reducingheat-related degradation of the exhaust components. For example, thepower EGR mode may enable the engine torque to be increased above thefirst threshold torque while maintaining the exhaust temperature belowthe threshold exhaust temperature. For example, providing EGR whileoperating in the power EGR mode enables maximum engine load/torque/powerby cooling the exhaust (dilution) and by increasing the knock limit,which further cools the exhaust. This cooling of the exhaust enablesmore air/load and thereby more power/torque output while maintaininglower exhaust temperatures, thereby decreasing the heat-relateddegradation of the exhaust components.

Transitioning the engine to the power EGR mode includes increasing theEGR rate over a plurality of engine cycles while operating all cylindersof the engine at stoichiometry, as indicated at 508. For example, athigher engine speeds and loads, such as may occur when the torque demandis approaching the first threshold torque, the engine may be operatedwithout EGR (e.g., with an EGR rate of zero). Therefore, the EGR ratemay be increased from zero responsive to the torque demand surpassingthe first threshold torque. As one example, the EGR rate may begradually increased from zero until the engine torque increases to thedriver demanded torque or the EGR rate reaches a threshold EGR rate. Thethreshold EGR rate may be a non-zero, pre-calibrated EGR rate abovewhich further increasing the EGR rate may result in misfire or partialburns. Increasing the EGR rate over the plurality of engine cycles mayinclude the controller gradually opening the EGR valve from a fullyclosed position at a pre-calibrated rate, such as by adjusting a controlsignal sent to an actuator of the EGR valve at the pre-calibrated rate,for example. At the same time, the boost pressure may be graduallyincreased, such as increased above the temperature-limited boostpressure threshold. For example, the boost pressure may be graduallyincreased at a rate proportional to a rate of EGR increase. By graduallyincreasing the EGR rate over the plurality of engine cycles, enginevibrations may be decreased and the heat-related degradation of exhaustcomponents may be further decreased relative to instantaneously steppingthe EGR rate to a desired rate for producing the corresponding powerincrease.

Further, the engine may continue to be operated at stoichiometry, withall of the cylinders operated with a same (e.g., uniform) AFR. Asmentioned above with respect to FIG. 2, operating the engine atstoichiometry may include small AFR fluctuations about stoichiometrybetween firings, but this is different than operating the engineglobally enriched, globally enleaned, or assigning different AFRs todifferent cylinders or cylinder groups (as in the split lambda operatingmode described herein).

Transitioning the engine to the power EGR mode further includesadjusting the spark timing based on the EGR rate, as indicated at 510.As one example, the spark timing may be globally adjusted (e.g., a sameadjustment is made for each engine cylinder) based on the EGR rate asthe EGR is gradually increased. Because EGR decreases the combustiontemperature, the spark timing may be advanced, with a degree of theadvancement proportional to the increase in the EGR rate. As oneexample, the controller may input the EGR rate (or the degree of openingof the EGR valve) into a look-up table, algorithm, or map, which mayoutput the corresponding spark timing. As another example, thecontroller may determine the spark timing based on logic rules that area function of the EGR rate (or the degree of opening of the EGR valve).The controller may then generate a control signal (e.g., signal SA) thatis sent an ignition system (e.g., ignition system 88 of FIG. 1) toactuate the spark plug of each cylinder at the determined spark timing.As an example, spark may be delivered at MBT spark timing or borderlinespark timing (e.g., based on feedback from a knock sensor).

At 512, method 500 includes determining whether the torque demand isgreater than a second threshold torque, which is greater than the firstthreshold torque described above at 504. The second threshold torque maybe a pre-calibrated non-zero engine torque value above which the torquecannot be further increased while operating the engine in the power EGRmode, such as due to reaching the threshold EGR rate. As an alternativeexample of the method at 512, it may be determined if the engine powerdemand is greater than a second threshold power, which may correspond tothe second threshold torque at a given engine speed.

If the torque demand is not greater than the second threshold torque,method 500 proceeds to 532 and includes operating the engine in thepower EGR mode. That is, EGR may be provided for exhaust componenttemperature relief, enabling the engine to produce torque values thatare greater than the first threshold torque and less than the secondthreshold torque without degrading exhaust components, as describedabove at 506. Thus, the engine may be transitioned into (e.g., at 506)and then operated in the power EGR mode (e.g., at 532).

If instead the torque demand is greater than the second thresholdtorque, method 500 proceeds to 514 and includes transitioning the engineinto the split lambda operating mode. As a first example, the engine maybe transitioned into the split lambda operating mode responsive to afurther increase in the torque demand that results in the torque demandsurpassing the second threshold torque while operating in the power EGRmode. As a second example, the torque demand may be rapidly increasedfrom below the first threshold torque to above the second thresholdtorque. In the second example, transitioning into the power EGR mode mayprepare the engine for transitioning into the split lambda operatingmode with reduced vibrations and increased exhaust component temperaturecontrol. For example, the engine may be transitioned into the splitlambda operating mode responsive to the EGR rate reaching the thresholdEGR rate while temporarily operating in the power EGR mode. Thus, thepower EGR mode serves as both a distinct operating mode for increasingengine power, such as when the torque demand is greater than the firstthreshold torque but less than the second threshold torque (e.g., as at532), and for transitioning the engine into the split lambda operatingmode.

Transitioning the engine into the split lambda operating mode includesoperating a first cylinder set at a first, rich AFR and a secondcylinder set at a second, lean AFR, as indicated at 516. In someexamples, the method may further include operating a third cylinder setat stoichiometry, such as when the engine includes an odd number ofcylinders. Each of the first cylinder set and the second cylinder setmay include one or more cylinders, with a number of cylinders in thefirst cylinder set equal to a number of cylinders in the second cylinderset. For example, when the engine includes an even number of cylindersand two exhaust manifolds, the cylinders may be equally divided betweenthe rich and lean cylinders (e.g., half of the cylinders are operated atthe rich AFR, and half of the cylinders are operated at the lean AFR).This may include operating all of the cylinders coupled to a firstexhaust manifold (e.g., first exhaust manifold 81 of FIGS. 2 and 3) atthe lean AFR and operating all of the cylinders coupled to a secondexhaust manifold (e.g., second exhaust manifold 85 of FIGS. 2 and 3) atthe rich AFR, the second exhaust manifold further coupled to the EGRsystem. As one example, when the engine includes two engine banks, thecylinders of a first engine bank (e.g., first engine bank 312 of FIG. 3)may be operated at the lean AFR while the cylinders of a second enginebank (e.g., second engine bank 314 of FIG. 3) may be operated at therich AFR. As another example, when the engine includes an odd number ofcylinders, one cylinder may be operated at stoichiometry while theremaining cylinders are equally divided between the rich AFR (e.g., thefirst cylinder set) and the lean AFR (e.g., the second cylinder set).Further, as elaborated above with respect to FIG. 2, the rich AFR andthe lean AFR may be balanced to produce a stoichiometric mixture at thedownstream emission control device.

Further still, transitioning the engine into the split lambda modeincludes increasing a difference between the rich AFR and the lean AFRwhile maintaining or decreasing the EGR rate over a plurality of enginecycles, as indicated at 518. The difference between the rich AFR and thelean AFR may be referred to herein as a lambda split. Thus,transitioning the engine into the split lambda mode includes increasingthe lambda split. For example, the lambda split may be incrementallyincreased while the EGR rate is incrementally decreased each enginecycle until the engine demand is met. This may include the controllerfurther enriching the first set of cylinders each engine cycle andfurther enleaning the second set of cylinders by a corresponding amountto maintain a stoichiometric exhaust gas mixture at the emission controldevice. For example, a degree of the enrichment of the first set ofcylinders may be equal to a degree of the enleanment of the second setof cylinders each engine cycle. As one non-limiting illustrativeexample, the rich AFR may be decreased by a lambda value of 0.02 eachengine cycle, and the lean AFR may be correspondingly increased eachengine cycle to maintain a stoichiometric exhaust gas mixture at theemission control device. As another example, the EGR rate may bemaintained for a plurality of engine cycles as the lambda split isincreased. As will be illustrated with respect to FIG. 6, at higherlambda splits and some EGR ranges, adjusting the EGR rate may not affectthe output engine torque/power.

As an example, the controller may determine an AFR to command for eachcylinder each engine cycle based on whether the cylinder is in the firstcylinder set or the second cylinder set (and in some examples, the thirdcylinder set) and a desired lambda split. Then, the controller mayadjust a pulse width of a signal FPW sent to a fuel injector of eachcylinder based on the commanded AFR of the particular cylinder and acylinder air charge amount, such as via a look-up table or function.Further, the controller may adjust the control signal sent to the EGRvalve each engine cycle in order to proportionally decrease the openingof the EGR valve as the lambda split is increased. However, the EGRvalve may remain partially open during operation in the split lambdamode in order to provide a non-zero EGR rate.

It may be understood that in order to meet the increased engine torquedemand, the boost pressure may be further increased. As an example, theboost pressure may be incrementally increased each engine cycle at arate proportional to the change in the lambda split each engine cycle.For example, the boost increase may be calibrated to the degree of theenrichment of the first set of cylinders as well as the EGR rate, twoparameters that influence exhaust component cooling, in order to preventand/or reduce heat-related degradation as the engine torque output isfurther increased.

Transitioning the engine into the split lambda operating mode furtherincludes adjusting the spark timing of each cylinder individually, asindicated at 520. Alternatively, this may include adjusting the sparktiming of each cylinder set independently from the other cylinderset(s). Because EGR decreases the combustion temperature and further dueto the EGR being drawn from the first, enriched cylinder set, the sparktiming may be advanced for each cylinder, with a degree of theadvancement different for the first cylinder set and the second cylinderset. Additionally, a degree of enrichment of the EGR may be equal to adegree of enrichment of the first cylinder set. Therefore, as oneexample, the controller may input the EGR rate (or the degree of openingof the EGR valve), the degree of enrichment of the EGR, and thecommanded AFR of the particular cylinder into a look-up table,algorithm, or map, which may output the corresponding spark timing. Asanother example, the controller may determine the spark timing based onlogic rules that are a function of the EGR rate (or the degree ofopening of the EGR valve), the degree of enrichment of the EGR, and thecommanded AFR of the cylinder. The controller may then generate acontrol signal that is sent the ignition system to actuate the sparkplug of each cylinder at the determined spark timing for that individualcylinder. As one example, the controller may store pre-calibrated MBTspark timing values for each cylinder set (e.g., rich, lean, andstoichiometry) in non-transitory memory, and each cylinder may beoperated at either MBT spark timing for that set of cylinders orborderline spark timing, as controlled by the knock sensor. Further, theMBT spark timing may be more advanced for the lean cylinder set and lessadvanced for the rich cylinder set due to different burn rates of thecylinder sets.

At 522, method 500 includes operating the engine in the split lambdaoperating mode. Thus, the engine may be transitioned into (e.g., at 514)and then operated in the split lambda mode (e.g., at 522) to provideexhaust component temperature relief with reduced vehicle emissions,enabling the engine to produce torque values that are greater than thesecond threshold torque while maintaining the exhaust temperature belowthe threshold exhaust temperature.

At 524, method 500 includes determining whether the torque demandremains above the second threshold torque (as defined above at 512). Forexample, the controller may continuously evaluate the engine operatingconditions to determine if there is a change in the torque demand, andresponsive to a change in the torque demand, compare the newly requestedtorque demand to the second threshold torque. As one example, after theengine is transitioned into the split lambda operating mode, the torquedemand may decrease below the second threshold torque, such as due to adriver tip-out event. If instead the torque demand remains above thesecond threshold torque, method 500 may return to 522 to continueoperating the engine in the split lambda mode.

If the torque demand decreased below the second threshold torque, method500 proceeds to 526 and includes transitioning the engine out of thesplit lambda operating mode. Similar to the sequence of engineadjustments for transitioning the engine into the split lambda operatingmode (e.g., described above at 514), the sequence of engine adjustmentsfor transitioning the engine out of the split lambda operating mode maybe calibrated to reduce engine vibrations and maintain the exhausttemperature below the threshold temperature.

Transitioning the engine out of the split lambda operating mode includesdecreasing the difference between the rich AFR and the lean AFR whilemaintaining or increasing the EGR rate over a plurality of enginecycles, as indicated at 528. For example, the lambda split may begradually decreased over the plurality of engine cycles until the lambdasplit reaches zero and all of the engine cylinders are operated at auniform commanded AFR (e.g., stoichiometry). This may include thecontroller decreasing the degree of enrichment of the first set ofcylinders each engine cycle and adjusting the lean AFR of the second setof cylinders by a corresponding amount to maintain the stoichiometricexhaust gas mixture at the emission control device. As one non-limitingillustrative example, the rich AFR may be increased by a lambda value of0.02 each engine cycle, with the lean AFR correspondingly increased. Atthe same time, the EGR rate may be gradually increased over theplurality of engine cycles until the threshold EGR rate is reached. Asanother example, the EGR rate may be maintained for a plurality ofengine cycles as the lambda split is decreased, as adjusting the EGRrate may not affect the output engine torque/power over some lambdasplits and EGR ranges.

Transitioning the engine out of the split lambda operating mode furtherincludes adjusting the spark timing for each cylinder individually, asindicated at 530. Alternatively, this may include adjusting the sparktiming of each cylinder set independently from the other cylinderset(s). Because the EGR rate, the degree of enrichment of the EGR, andthe commanded AFR of the first cylinder set and the second cylinder setare all changing each engine cycle during the transitioning, thecontroller may adjust the spark timing for each cylinder each enginecycle. Although the directionality of the adjustments made whiletransitioning the engine out of the split lambda mode may be oppositethose made while transitioning the engine into the split lambda mode(e.g., at 514), the controller may determine the spark timing for eachcylinder in the same manner described above at 520.

Once the engine has been transitioned out of the split lambda operatingmode and the engine is operated with a uniform commanded AFR (e.g.,stoichiometry), method 500 proceeds to 532 to operate the engine in thepower EGR mode, as described above. Thus, transitioning the engine outof the split lambda operating mode also includes transitioning theengine into the power EGR mode.

At 534, method 500 includes determining if the torque demand is greaterthan the first threshold torque (introduced above at 504). As oneexample, the torque demand may decrease below the second thresholdtorque (as determined at 524) while remaining above the first thresholdtorque, and thus, method 500 may return to 532 to continue operating inthe power EGR mode. If instead the torque demand has decreased below thefirst threshold torque, method 500 proceeds to 536 and includestransitioning the engine out of the power EGR mode.

Transitioning the engine out of the power EGR mode includes decreasingthe EGR rate over a plurality of engine cycles while operating all ofthe cylinders at stoichiometry, as indicated at 538. As one example, theEGR rate may be decreased at the same rate the EGR was increased at 508.For example, the controller may decrease the EGR rate from the thresholdEGR rate over a plurality of engine cycles by gradually closing the EGRvalve until the EGR valve is fully closed and the EGR rate is zero. Bygradually decreasing the EGR rate over the plurality of engine cycles,engine vibrations may be decreased and the heat-related degradation ofexhaust components may be further decreased relative to instantaneouslyreducing the EGR rate to zero.

Transitioning the engine out of the power EGR mode further includesadjusting the spark timing based on the EGR rate, as indicated at 540.As one example, the spark timing may be globally adjusted (e.g., thesame adjustment is made for each engine cylinder) based on the EGR rateas the EGR is gradually decreased. Because EGR decreases the combustiontemperature, the spark timing may be less advanced as the EGR ratedecreases each engine cycle. Although the directionality of theadjustments made while transitioning the engine out of the power EGRmode may be opposite those made while transitioning the engine into thepower EGR mode (e.g., at 506), the controller may determine the sparktiming as described above at 510.

Once the engine is transitioned out of the power EGR mode, method 500may proceed to 542 to operate the engine in the stoichiometric mode, asdescribed above. For example, in response to the torque demandcontinuing to decrease and the engine load dropping below the thresholdload, the EGR rate may be increased from zero for increased engineefficiency and decreased fuel consumption during part-load operation.Further, boost may be provided based on engine demand while remainingbelow the temperature-limited boost pressure threshold.

In this way, method 500 provides a method for operating an engine of avehicle in different operating modes (e.g., a stoichiometric mode, apower EGR mode, and a split lambda mode) based on an engine torque (orpower) demand while reducing heat-related exhaust component degradationand reducing vehicle emissions. As illustrated by examples herein, themethod of operating and performing actions responsive to a determinationof the engine torque demand may include operating the engine to producetorque (e.g., operating with the vehicle traveling and the enginecombusting to provide driver-demanded torque), selecting an operatingmode that will provide the engine torque demand (such as based on sensoroutput, e.g., accelerator pedal position sensor output) and performingactions in response thereto. For example, in response to the enginetorque demand being less than a first threshold, a controller may selectthe stoichiometric operating mode. Responsive to selecting thestoichiometric operating mode, the controller may set a commanded AFR tostoichiometry for all cylinders and actuate fuel injectors to fuel theengine accordingly to operate the engine in the stoichiometric mode. Asanother example, in response to the engine torque demand being greaterthan the first threshold and less than a second, higher threshold, thecontroller may select the power EGR mode. Responsive to selecting thepower EGR mode, the controller may transition the engine into the powerEGR mode by increasing the EGR rate over a plurality of engine cycles(e.g., by adjusting a signal sent to an EGR valve to increase an openingof the EGR valve) and advancing the spark timing (e.g., by adjusting aspark advance signal sent to an ignition system). Further, thecontroller may set the commanded AFR at stoichiometry for all cylinders.As still another example, in response to the engine torque demandexceeding the second threshold, the controller may select the splitlambda mode. Responsive to selecting the split lambda mode, thecontroller may transition the engine into the split lambda mode bydecreasing the EGR rate (to a non-zero EGR rate), adjusting the AFR of afirst cylinder set to a rich AFR, and adjusting the AFR of a secondcylinder set to a lean AFR, a degree of enrichment of the rich AFR and adegree of the enleanment of the second cylinder set increased over aplurality of engine cycles and set to produce an overall stoichiometricAFR. Additionally, the controller may further advance the spark timing.

The transitioning into and out of the power EGR mode and the splitlambda mode described above is illustrated with respect to FIG. 6. Anexample graph 600 of FIG. 6 shows a relationship between engine powerand EGR rate at different commanded AFRs and spark timings. Thehorizontal axis represents the EGR rate, with the EGR rate increasingfrom zero along the horizontal axis from left to right. The verticalaxis represents the engine power, with the engine power increasing upthe vertical axis from bottom to top. Further, each plot of graph 600represents different engine operating parameters. Plot 602 shows engineoperation at stoichiometry without optimized spark timing, plot 604shows engine operation at stoichiometry with optimized spark timing,plot 606 shows engine operation with a first lambda split and optimizedspark timing, plot 608 shows engine operation with a second lambda splitand optimized spark timing, plot 610 shows engine operation with a thirdlambda split and optimized spark timing, plot 612 shows engine operationwith a forth lambda split and optimized spark timing, and 614 showsengine operation with a fifth lambda split and optimized spark timing.Further, an EGR threshold (EGR_T) is shown, which may be the thresholdEGR rate described above with respect to 508 of FIG. 5. Two engine powerthresholds are shown, a first, lower engine power threshold (Pwr_T1) anda second, higher engine power threshold (Pwr_T2). The first engine powerthreshold may correspond to the first engine torque threshold describedabove with respect to 504 of FIG. 5 for a given engine speed, and thesecond engine power threshold may correspond to the second engine torquethreshold described above with respect to 512 of FIG. 5 for the givenengine speed.

As illustrated by plots 602 and 604, optimizing the spark timing whileoperating the engine at stoichiometry increases engine power. Forexample, for a same EGR rate, plot 604, representing engine operation atstoichiometry with optimized spark timing, results in a higher enginepower than when the engine is operated at stoichiometry withoutoptimized spark timing (plot 602). Further, each of plots 602 and 604show that increasing the EGR rate results in increased engine power. Forexample, in response to an engine power demand surpassing the firstengine power threshold, the EGR rate may be increased from zero toincrease the engine power, as indicated by dashed arrow 616. Thus,dashed arrow 616 generally represents phasing in EGR duringtransitioning into the power EGR mode, as described above with respectto 506 of FIG. 5.

If the engine power demand remains between the first engine powerthreshold and the second engine power threshold, the engine may beoperated in the power EGR mode, such as along plot 604. If instead theengine power demand is greater than the second engine power thresholdand/or the EGR rate reaches the EGR threshold, the engine may betransitioned into the split lambda mode. Plots 606, 608, 610, 612, and614 all show engine operation in the split lambda mode, each having adifferent lambda split. In the example of graph 600, plot 606 includesthe smallest lambda split, and plot 614 includes the largest lambdasplit, with the lambda split gradually increasing across the plotsbetween plot 606 and plot 614 (e.g., plot 610 includes a greater lambdasplit than plot 608, and plot 612 includes a greater lambda split thanplot 610). As illustrative examples, plot 606 may include engineoperation with λ=0.93 for the rich AFR (e.g., in a first cylinder set),plot 608 may include engine operation with λ=0.91 for the rich AFR, plot610 may include engine operation with λ=0.89 for the rich AFR, plot 612may include engine operation with λ=0.87 for the rich AFR, and plot 614may include engine operation with λ=0.85 for the rich AFR. Further, eachlambda split includes engine operation with a corresponding lean AFR(e.g., in a second cylinder set) so that a stoichiometric exhaust gasmixture is produced by the engine. As demonstrated by these plots, agreater lambda split results in greater engine power for a same EGRrate.

As described above with respect to 514 of FIG. 5, transitioning into thesplit lambda mode includes increasing the lambda split while decreasingthe EGR rate, as indicated by a dashed arrow 618. Thus, dashed arrow 618generally represents phasing in the lambda split while phasing out EGR(to a non-zero rate). As one example, the engine power demand may beincreased from below the first engine power threshold to above thesecond engine power threshold. In such an example, adjustments to theengine operating parameters may first follow dashed arrow 616 and thenfollow dashed arrow 618. Based on the particular engine power demandwhile operating in the split lambda mode, the engine may be operatedalong one of plots 606, 608, 610, 612, and 614, for example, althoughother plots are also possible (e.g., having different lambda splits).

Transitioning out of the split lambda mode to the EGR power mode isshown generally by a dotted arrow 620. As elaborated above with respectto 526 of FIG. 5, responsive to the engine power demand decreasing belowthe second engine power threshold the lambda split is decreased (e.g.,until the lambda split equals zero) while the EGR rate is increased,resulting in the engine transitioning into the power EGR mode. If theengine power demand is further below the first engine power threshold,the engine is further transitioned out of the power EGR mode (and intothe stoichiometric mode), as generally shown by a dotted arrow 622. Forexample, the EGR rate is gradually decreased, in the direction of dottedarrow 622, over a plurality of engine cycles, as elaborated above withrespect to 536 of FIG. 5, until the EGR rate is equal to zero. It may beunderstood that engine operation in the stoichiometric mode is notillustrated in graph 600, as EGR may not be used to increase enginepower in the stoichiometric mode.

Turning now to FIG. 7, an example timeline 700 shows transitioning anengine between different operating modes responsive to a changing enginetorque demand. The engine may be engine 10 included in any of the enginesystem configurations shown in FIGS. 2-4, for example, that enablerecirculation of enriched EGR during split lambda operation for furtherincreased engine power. The engine torque demand is shown in plot 702, aboost pressure provided by a turbocharger is shown in plot 704, anoperating mode is shown in plot 706, a lambda split is shown in plot708, an EGR rate is shown in plot 710, an exhaust temperature is shownin plot 712, a delivered spark timing advance of a first set of enginecylinders is shown in plot 714, and a delivered spark timing advance ofa second set of engine cylinders is shown in dashed plot 716. Further, afirst, lower engine torque threshold is shown by dashed line 720, asecond, higher engine torque threshold is shown by dashed line 722, atemperature-limited boost pressure threshold is shown by dashed line724, a threshold EGR rate is shown by dashed line 726, and a thresholdexhaust temperature is shown by dashed line 728.

For all of the above, the horizontal axis represents time, with timeincreasing along the horizontal axis from left to right. The verticalaxis represents each labeled parameter. For plots 702, 704, 708, 710,712, 714, and 716, a magnitude of the parameter increases up thevertical axis from bottom to top. For plot 706, the vertical axis showswhether the engine is operating in a stoichiometric mode (“stoich”), apower EGR mode (“power EGR”), or a split lambda mode (“split lambda”),as labeled, which correspond to the stoichiometric mode, the power EGRmode, and the split lambda mode described above with respect to FIG. 5.Further, the boost pressure may be understood to include intake manifoldpressures that are greater than atmospheric pressure. For example, whenthe boost pressure is zero, the engine may be operating without boost(e.g., via natural aspiration), such as at lower engine speeds andloads. Additionally, the example of timeline 700 does not show responsedelays, such as a delay between a request for increased engine torqueand a corresponding increase in the boost pressure. Further, sparktimings for two cylinder sets are shown, corresponding to an enginehaving an even number of cylinders (e.g., as in engine system 200 ofFIG. 2 and engine system 300 of FIG. 3). However, when the engineinstead includes an odd number of cylinders (e.g., as in engine system400 of FIG. 4), the spark timing for a third cylinder set also may beincluded.

Prior to time t1, the engine torque demand (plot 702) is less than thefirst threshold torque (dashed line 720). As a result, the engine isoperated in the stoichiometric mode (plot 706), with a lambda split ofzero (plot 708). While the engine is operated in the stoichiometricmode, both the EGR rate (plot 710) and the boost pressure (plot 704) areadjusted based on the engine torque demand. For example, when the enginetorque demand (plot 702) is low, such as when the engine is operated atidle, EGR is not provided, and the EGR rate is zero. To maintain the EGRrate at zero, an EGR valve (e.g., EGR valve 54 of FIGS. 2-4) is held ata fully closed position. Then, as the engine torque demand increases,the EGR rate is increased from zero (plot 710) by increasing an openingof the EGR valve from the fully closed position. As the engine torquedemand further increases and begins to approach the first thresholdtorque (dashed line 720), the EGR rate is again decreased to zero (plot710) by decreasing the opening of the EGR valve until the EGR valve isfully closed. Similarly, when the engine torque demand (plot 702) islow, boost may not be provided, with a boost pressure of zero (plot704). The boost pressure (plot 704) is increased as the engine torquedemand increases (plot 702) in order to provide compressed air to meetthe increased engine torque demand. Further, the delivered spark timingis the same for the first cylinder set and the second cylinder set(e.g., solid plot 714 and dashed plot 716 are overlapping) due to engineoperation with a uniform AFR (e.g., stoichiometry) and varies based on,for example, engine speed and load.

As the EGR rate (plot 710) is decreased and the boost pressure (plot704) is increased, the exhaust temperature increases (plot 712) andapproaches the threshold exhaust temperature (dashed line 726). Attemperatures above the threshold exhaust temperature, heat-relateddegradation of exhaust components, such as a turbine of the turbochargerand an emission control device, may occur. This corresponds to the boostpressure (plot 704) approaching the temperature-limited boost pressurethreshold (dashed line 724). The temperature-limited boost pressurethreshold corresponds to a maximum amount of boost that may be providedwhile operating in the stoichiometric mode (plot 706) in order tomaintain the exhaust temperature (plot 712) below the threshold exhausttemperature (dashed line 728). Because the boost is limited by theexhaust temperature, the amount of torque the engine can produce whileoperating in the stoichiometric mode is also limited to below the firstthreshold torque (dashed line 720).

At time t1, responsive to the engine torque demand (plot 702) surpassingthe first threshold torque (dashed line 720), the engine is transitionedto the power EGR mode (plot 706) in order to provide exhaust temperaturerelief via cooled EGR. Thus, the EGR rate (plot 710) is graduallyincreased from zero (e.g., by gradually opening the EGR valve from thefully closed position). All of the engine cylinders continue to beoperated at stoichiometry in the power EGR mode, and so the lambda splitremains equal to zero (plot 708). The spark timing for each cylinder isoperated at MBT timing due to the cooling effect of the EGR and may beapproximately the same for each cylinder set due to the uniformcommanded AFR (plots 714 and 716). Additionally, due to the coolingeffect of the EGR, the boost pressure (plot 704) is further increasedabove the temperature-limited boost pressure threshold (dashed line724), enabling the engine torque to meet the engine torque demand whilethe exhaust temperature (plot 712) decreases from and remains below theexhaust temperature threshold (dashed line 728). By maintaining all ofthe cylinders at stoichiometry, the emission control device continues toefficiently treat exhaust gas, resulting in decreased vehicle emissions,while the engine torque is increased and engine vibrations areminimized.

Between time t1 and time t2, the engine torque demand (plot 702) remainsgreater than the first threshold torque (dashed line 720) and less thanthe second threshold torque (dashed line 722). Therefore, the engine isoperated in the power EGR mode (plot 706) between time t1 and time t2.Further, the EGR rate (plot 710) and the boost pressure (plot 704) areadjusted based on the engine torque demand (plot 702), such as bydecreasing the EGR rate and the boost pressure responsive to the enginetorque demand decreasing while remaining between the first thresholdtorque and the second threshold torque.

At time t2, the engine torque demand (plot 702) decreases below thefirst threshold torque (dashed line 720). In response, the engine istransitioned out of the power EGR mode and into the stoichiometric mode(plot 706). To transition the engine out of the power EGR mode, the EGRrate is gradually decreased to zero (plot 710), enabling the exhausttemperature (plot 712) to remain below the threshold exhaust temperature(dashed line 728). At the same time, the boost pressure (plot 704) isdecreased below the temperature-limited boost pressure threshold (dashedline 724). Upon transitioning out of the power EGR mode and into thestoichiometric mode, the engine is operated in the stoichiometric mode(plot 706) while the engine torque demand (plot 702) remains below thefirst threshold torque (dashed line 720) between time t2 and time t3.Additionally, with the engine operating in the stoichiometric mode, thelambda split remains at zero (plot 708).

At time t3, the engine torque demand (plot 702) again increases abovethe first threshold torque (dashed line 720). In response, the enginebegins transitioning into the power EGR mode (plot 706), with the EGRrate gradually increased from zero (plot 710). While this transition isoccurring, the engine torque demand (plot 702) continues to increase andsurpasses the second torque demand threshold (dashed line 722). Thus,the engine transitions into the split lambda mode (plot 706). Forexample, once the EGR rate (plot 710) reaches the threshold EGR rate(dashed line 726), the EGR rate is gradually decreased while the lambdasplit (plot 708) is gradually increased. The first set of cylinders isoperated at a rich AFR and the second set of cylinders is operated at alean AFR, with a degree of enrichment of the rich AFR graduallyincreased as the lambda split is phased in and the lean AFR adjustedaccordingly to produce a stoichiometric exhaust mixture at the emissioncontrol device. Further, the spark timing advance is adjusteddifferently for the different cylinder sets. The delivered spark timingadvance is lower for the first, rich cylinder set (plot 714) and higherfor the second, lean cylinder set (plot 716), although both cylindersets may be operated at MBT timing (or, alternatively, borderline sparktiming) for the corresponding cylinder set.

While the engine is operated in the split lambda mode, the boostpressure (plot 704) is increased above the temperature-limited boostpressure threshold (dashed line 724), enabling the engine torque to meetthe engine torque demand while the exhaust temperature (plot 712)remains below the exhaust temperature threshold (dashed line 728).Further, the lambda split and/or the EGR rate may be adjusted based onthe engine torque demand, such as by decreasing the lambda split and/orthe EGR rate in response to a decreased engine torque demand (thatremains above the second threshold torque) and increasing the lambdasplit and/or the EGR rate in response to an increased engine torquedemand. As shown between time t3 and time t4, when the lambda split(plot 708) is relatively high, the EGR rate (plot 710) is heldrelatively constant, even while there are slight fluctuations in enginetorque demand (plot 702).

At time t4, the engine torque demand (plot 702) decreases below thesecond threshold torque (dashed line 722). As a result, the engine istransitioned out of the split lambda mode and into the power EGR mode.Transitioning out of the split lambda mode and into the power EGR modeincludes gradually increasing the EGR rate (plot 710) over a pluralityof engine cycles until the threshold EGR rate (dashed line 726) isreached. Further, the lambda split (plot 708) is decreased over theplurality of engine cycles until the engine is operated with a uniformAFR and the lambda split reaches zero. Due to the decreased enginetorque demand, the boost pressure is decreased (plot 704) but remainsabove the temperature-limited boost pressure threshold (dashed line724). Due to the gradual decrease of the lambda split and the gradualincrease of the EGR rate during the transition, the exhaust temperature(plot 712) remains below the exhaust temperature threshold (dashed line728). Further, each cylinder set is operated with approximately the samedelivered spark timing advance (plots 714 and 716) due to transitioningto a uniform commanded AFR.

Following the transition out of the split lambda operating mode, theengine torque demand (plot 702) remains above the first torque threshold(dashed line 720). Therefore, the engine is operated in the power EGRmode (plot 706). However, the engine torque demand (plot 702) thendecreases below the first torque threshold (dashed line 720) at time t5,and so the engine is transitioned out of the power EGR mode and into thestoichiometric mode (plot 706). This includes gradually decreasing theEGR rate until the EGR rate reaches zero (plot 710). The boost pressure(plot 704) is decreased below the temperature-limited boost pressurethreshold (dashed line 724) so that the exhaust temperature (plot 712)remains below the threshold exhaust temperature (dashed line 728).Further still, the lambda split remains at zero (plot 708).

At time t6, the engine torque demand (plot 702) again increases abovethe first torque threshold (dashed line 720). In response, the engine istransitioned to operating in the power EGR mode (plot 706), with the EGRrate gradually increased from zero (plot 710) while the lambda splitremains at zero (plot 708). Due to the cooling effect of the EGR, theboost pressure (plot 704) is increased above the temperature-limitedboost pressure threshold (dashed line 724), enabling the engine torqueto meet the higher engine torque demand while the exhaust temperature(plot 712) remains below the exhaust temperature threshold (dashed line728). The spark timing for each cylinder is also further advancedcompared to spark timing outside of the power EGR mode for the sameengine speed and load (plots 714 and 716).

The engine is operated in the power EGR mode between time t5 and timet6. However, at time t6, the engine torque demand (plot 702) increasesabove the second torque threshold (dashed line 722). In response, theengine is transitioned to operating in the split lambda mode (plot 706).During the transitioning, the EGR rate (plot 710) is gradually increasedto the threshold EGR rate (dashed line 726) and then gradually decreasedwhile the lambda split (plot 708) is gradually increased. The first setof cylinders is operated at a rich AFR and the second set of cylindersis operated at a lean AFR, with the degree of enrichment of the rich AFR(and the degree of enleanment of the lean AFR) gradually increased asthe lambda split is phased in. The third set of cylinders (when theengine includes an odd number of cylinders) is maintained atstoichiometry. Further, the spark timing advance is adjusted differentlyfor the different cylinder sets, as described above with respect to thesplit lambda transition between time t3 and time t4. While the engine isoperated in the split lambda mode, the boost pressure (plot 704) isstill further increased above the temperature-limited boost pressurethreshold (dashed line 724), enabling the engine torque to meet theengine torque demand while the exhaust temperature (plot 712) remainsbelow the exhaust temperature threshold (dashed line 728).

At time t7, the engine torque demand (plot 702) decreases below thesecond torque threshold (dashed line 722), and so the engine istransitioned out of the split lambda mode, with the EGR rate (plot 710)gradually increased over a plurality of engine cycles until thethreshold EGR rate (dashed line 726) is reached. However, during thetransition out of the split lambda mode, the engine torque demand (plot702) continues to decrease and decreases below the first torquethreshold (dashed line 720). Thus, the engine transitions into thestoichiometric mode (plot 706), and the power EGR mode serves as atransitory mode during the transition between the split lambda mode andthe stoichiometric mode. Further, the lambda split (plot 708) isdecreased over the plurality of engine cycles until the engine isoperated with a uniform AFR and the lambda split reaches zero, and thenthe EGR rate is decreased to zero (plot 710). Due to the decreasedengine torque demand and also due to cooling EGR and enrichment notbeing provided, the boost pressure is decreased (plot 704) below thetemperature-limited boost pressure threshold (dashed line 724). Due tothe gradual decrease of the lambda split and the gradual increase of theEGR rate during the transition, the exhaust temperature (plot 712)remains below the exhaust temperature threshold (dashed line 728).

In this way, an engine of a vehicle may be configured to flow EGR from aspecific subset of cylinders and may be operated in different operatingmodes (e.g., a stoichiometric mode, a power EGR mode, and a split lambdamode) based on an engine torque (or power) demand. In particular,operating in the power EGR mode and the split lambda mode enable engineairflow, and thus engine power, to be increased while reducingheat-related exhaust component degradation. Further, increasing enginepower via the power EGR mode and via the split lambda mode increasesengine power while reducing vehicle emissions compared with conventionalenrichment strategies. Additionally, the split lambda operating mode isenhanced via enriched EGR, which enables further spark timingadvancements for additional power gains. Further still, vibrations dueto imbalance between rich and lean cylinders while operating in thesplit lambda mode may be reduced by transitioning through the power EGRmode, which includes phasing in EGR, and then phasing out EGR whilephasing in a lambda split.

The technical effect of partially enriching an engine while maintainingthe engine at global stoichiometry and recirculating enriched EGR isthat engine power may be increased without increasing vehicle emissions.

The technical effect of gradually increasing an EGR rate to increaseengine power and then gradually decreasing the EGR rate while increasingan air-fuel ratio difference between two groups of cylinders to furtherincrease the engine power, the air-fuel ratio difference providingpartial engine enrichment while maintaining the engine at globalstoichiometry, is that engine vibrations may be reduced while exhaustcomponent temperatures are maintained below an upper threshold.

As one example, a method comprises: while operating an engine, enrichinga first set of cylinders, enleaning a second set of cylinders, andmaintaining a third set of cylinders at stoichiometry, exhaust gas fromthe first set, the second set, and the third set producing astoichiometric mixture at a downstream emission control device, andproviding exhaust gas recirculation (EGR) to an intake passage of theengine from the first set of cylinders. In the preceding example,additionally or optionally, enriching the first set of cylinders,enleaning the second set of cylinders, and maintaining the third set ofcylinders at stoichiometry is responsive to an engine torque demandbeing greater than a threshold torque and further responsive to a rateof the EGR of the reaching a threshold rate. In one or both of thepreceding examples, the method additionally or optionally furthercomprises, responsive to the rate of the EGR remaining below thethreshold rate while the engine torque demand is greater than thethreshold torque, maintaining the first set of cylinders, the second setof cylinders, and the third set of cylinders at stoichiometry whileincreasing the rate of the EGR responsive to an increase in the enginetorque demand. In any or all of the preceding examples, additionally oroptionally, the third set of cylinders includes one cylinder, the firstset of cylinders includes a first half of a total number of remainingcylinders in the engine, and the second set of cylinders includes asecond half of the total number of remaining cylinders in the engine. Inany or all of the preceding examples, additionally or optionally, an EGRpassage couples an exhaust runner of at least one of the first set ofcylinders to the intake passage of the engine, the EGR passage includingan EGR valve and an EGR cooler disposed therein, and wherein providingEGR to the intake passage of the engine from the first set of cylindersincludes at least partially opening the EGR valve. In any or all of thepreceding examples, additionally or optionally, the EGR passage iscoupled to the intake passage downstream of a compressor of aturbocharger, and a turbine of the turbocharger is coupled in an exhaustpassage upstream of the emission control device. In any or all of thepreceding examples, additionally or optionally, a boost pressureprovided by the turbocharger is increased above a threshold boostresponsive to enriching the first set of cylinders, enleaning the secondset of cylinders, and maintaining the third set of cylinders atstoichiometry. In any or all of the preceding examples, additionally oroptionally, the threshold boost is set based on a temperature of one ormore of the emission control device and the turbine of the turbocharger.In any or all of the preceding examples, the method additionally oroptionally further comprises adjusting the spark timing in the first setof cylinders to a first amount of spark advance, adjusting the sparktiming the second set of cylinders to a second amount of spark advance,different than the first amount of spark advance, and adjusting thespark timing in the third set of cylinders to a third amount of sparkadvance, different than each of the first amount of spark advance andthe second amount of spark advance. In any or all of the precedingexamples, additionally or optionally, the first amount of spark advanceis greater than the second amount of spark advance and the third amountof spark advance, and the third amount of spark advance is greater thanthe second amount of spark advance.

As another example, a method comprises: selecting between operating anengine in a stoichiometric mode, a power exhaust gas recirculation (EGR)mode, and a split lambda mode based on an engine torque demand;responsive to selecting the stoichiometric mode, operating the engine inthe stoichiometric mode, including operating all engine cylinders at astoichiometric air-fuel ratio (AFR); responsive to selecting the powerEGR mode, operating the engine in the power EGR mode, includingoperating all engine cylinders at the stoichiometric AFR whileincreasing an EGR rate; and responsive to selecting the split lambdamode, operating the engine in the split lambda mode, including operatinga first number of the engine cylinders rich, a second number of theengine cylinders lean, and a third number of the engine cylinders atstoichiometry while providing EGR from the first number of the enginecylinders only. In the preceding example, additionally or optionally,selecting between operating the engine in the stoichiometric mode, thepower EGR mode, and the split lambda mode based on the engine torquedemand includes: selecting the stoichiometric mode responsive to theengine torque demand being less than a first, lower threshold; selectingthe power EGR mode responsive to the engine torque demand being greaterthan the first threshold and less than a second, higher threshold; andselecting the split lambda mode responsive to the engine torque demandbeing greater than the second threshold. In one or both of the precedingexamples, additionally or optionally, operating the first number of theengine cylinders rich, the second number of the engine cylinders lean,and the third number of the engine cylinders at stoichiometry includesoperating the first number of the engine cylinders at a first, rich AFR,operating the second number of the engine cylinders at a second, leanAFR, and operating the third number of the engine cylinders atstoichiometry, a degree of enrichment of the first, rich AFR equal to adegree of enleanment of the second, lean AFR. In any or all of thepreceding examples, additionally or optionally, the degree of enrichmentis increased and the EGR rate is decreased as the engine torque demandfurther increases above the second threshold. In any or all of thepreceding examples, additionally or optionally, operating the engine inthe stoichiometric mode includes increasing engine torque by increasingan amount of boost provided by a turbocharger, operating the engine inthe power EGR mode includes increasing engine torque by increasing theEGR rate until a threshold rate is reached while further increasing theamount of boost provided by the turbocharger, and operating in the splitlambda mode includes increasing engine torque by increasing a degree ofenrichment of the first half of the engine cylinders while decreasingthe EGR rate and still further increasing the amount of boost providedby the turbocharger.

As another example, a system comprises: an engine, including an oddnumber of cylinders, each cylinder including an exhaust runner coupledto an exhaust manifold; and a controller with computer readableinstructions stored on non-transitory memory that, when executed duringengine operation, cause the controller to: operate a first set ofcylinders at a rich air-fuel ratio, operate a second set of cylinders ata lean air-fuel ratio, and operate a third set of cylinders atstoichiometry responsive to an engine demand greater than an upperthreshold demand, the first set of cylinders including the firstcylinder; and operate the first set of cylinders, the second set ofcylinders, and the third set of cylinders at a same air-fuel ratioresponsive to an engine demand less than the upper threshold demand. Inthe preceding example, the system additionally or optionally furthercomprises an exhaust gas recirculation (EGR) passage coupled between theexhaust runner of a cylinder of the first set of cylinders and an intakepassage of the engine, and the controller includes further computerreadable instructions stored in non-transitory memory that, whenexecuted during engine operation, cause the controller to: adjust theEGR valve to provide a non-zero amount of EGR that is less than athreshold amount of EGR while operating the first set of cylinders atthe rich air-fuel ratio, operating the second set of cylinders at a leanair-fuel ratio, and operating the third set of cylinders atstoichiometry. In one or both of the preceding examples, the systemadditionally or optionally further comprises an emission control devicecoupled in an exhaust passage downstream of the exhaust manifold, andoperating the first set of cylinders at the rich air-fuel ratio,operating the second set of cylinders at a lean air-fuel ratio, andoperating the third set of cylinders at stoichiometry produces astoichiometric air-fuel ratio at the emission control device. In any orall of the preceding examples, the system additionally or optionallyfurther comprises a turbocharger, including a compressor coupled to anintake of the engine and a turbine coupled in the exhaust passageupstream of the emission control device, and the controller includesfurther computer readable instructions stored in non-transitory memorythat, when executed during engine operation, cause the controller to:increase a boost pressure provided by the turbocharger above atemperature-limited boost pressure threshold while operating the firstset of cylinders at the rich air-fuel ratio, operating the second set ofcylinders at a lean air-fuel ratio, and operating the third set ofcylinders at stoichiometry. In any or all of the preceding examples, thesystem additionally or optionally further comprises a spark plug coupledto each cylinder, and the controller includes further computer readableinstructions stored in non-transitory memory that, when executed duringengine operation, cause the controller to: advance a spark timing of thespark plug coupled to each cylinder while operating the first set ofcylinders at the rich air-fuel ratio, operating the second set ofcylinders at a lean air-fuel ratio, and operating the third set ofcylinders at stoichiometry, a first amount of spark timing advance forthe first set of cylinders different than each of a second amount ofspark timing advance for the second set of cylinders and a third amountof spark timing advance for the third set of cylinders.

In another representation, a method comprises: operating an engine witha first set of cylinders enriched, a second set of cylinders enleaned,and a third set of cylinders at stoichiometry, a degree of enrichment ofthe first set of cylinders equal to a degree of enleanment of the secondset of cylinders; and while operating the engine with the first set ofcylinders enriched, the second set of cylinders enleaned, and the thirdset of cylinders at stoichiometry, recirculating exhaust gas to anintake passage of the engine from the first set of cylinders only andadvancing spark timing in each cylinder, the advanced spark timingindividually adjusted for each cylinder. In the preceding example,additionally or optionally, all cylinders of the engine are included inthe first set of cylinders, the second set of cylinders, and the thirdset of cylinders, and combined exhaust gas from all the cylinders of theengine produces a stoichiometric air-fuel ratio at a downstreamcatalyst. In one or both of the preceding examples, additionally oroptionally, the third set of cylinders includes one cylinder, the firstset of cylinders includes a first half of a remaining number ofcylinders, and the second set of cylinders includes a second half of theremaining number of cylinders. In any or all of the preceding examples,additionally or optionally, the first set of cylinders and the secondset of cylinders each include one cylinder. In any or all of thepreceding examples, additionally or optionally, an EGR passage iscoupled between an intake passage of the engine and an exhaust runner ofthe first cylinder. In any or all of the preceding examples,additionally or optionally, operating the engine with the first set ofcylinders enriched, the second set of cylinders enleaned, and the thirdset of cylinders at stoichiometry is responsive to an engine torquedemand greater than an upper threshold.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

As used herein, the term “approximately” is construed to mean plus orminus five percent of the range unless otherwise specified.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method, comprising: while operating an engine, enriching a firstset of cylinders, enleaning a second set of cylinders, and maintaining athird set of cylinders at stoichiometry, exhaust gas from the first set,the second set, and the third set producing a stoichiometric mixture ata downstream emission control device, and providing exhaust gasrecirculation (EGR) to an intake passage of the engine from the firstset of cylinders.
 2. The method of claim 1, wherein enriching the firstset of cylinders, enleaning the second set of cylinders, and maintainingthe third set of cylinders at stoichiometry is responsive to an enginetorque demand being greater than a threshold torque and furtherresponsive to a rate of the EGR of the reaching a threshold rate.
 3. Themethod of claim 2, further comprising: responsive to the rate of the EGRremaining below the threshold rate while the engine torque demand isgreater than the threshold torque, maintaining the first set ofcylinders, the second set of cylinders, and the third set of cylindersat stoichiometry while increasing the rate of the EGR responsive to anincrease in the engine torque demand.
 4. The method of claim 1, whereinthe third set of cylinders includes one cylinder, the first set ofcylinders includes a first half of a total number of remaining cylindersin the engine, and the second set of cylinders includes a second half ofthe total number of remaining cylinders in the engine.
 5. The method ofclaim 1, wherein an EGR passage couples an exhaust runner of at leastone of the first set of cylinders to the intake passage of the engine,the EGR passage including an EGR valve and an EGR cooler disposedtherein, and wherein providing EGR to the intake passage of the enginefrom the first set of cylinders includes at least partially opening theEGR valve.
 6. The method of claim 5, wherein the EGR passage is coupledto the intake passage downstream of a compressor of a turbocharger, anda turbine of the turbocharger is coupled in an exhaust passage upstreamof the emission control device.
 7. The method of claim 6, wherein aboost pressure provided by the turbocharger is increased above athreshold boost responsive to enriching the first set of cylinders,enleaning the second set of cylinders, and maintaining the third set ofcylinders at stoichiometry.
 8. The method of claim 7, wherein thethreshold boost is set based on a temperature of one or more of theemission control device and the turbine of the turbocharger.
 9. Themethod of claim 1, further comprising adjusting the spark timing in thefirst set of cylinders to a first amount of spark advance, adjusting thespark timing the second set of cylinders to a second amount of sparkadvance, different than the first amount of spark advance, and adjustingthe spark timing in the third set of cylinders to a third amount ofspark advance, different than each of the first amount of spark advanceand the second amount of spark advance.
 10. The method of claim 9,wherein the first amount of spark advance is greater than the secondamount of spark advance and the third amount of spark advance, and thethird amount of spark advance is greater than the second amount of sparkadvance.
 11. A method, comprising: selecting between operating an enginein a stoichiometric mode, a power exhaust gas recirculation (EGR) mode,and a split lambda mode based on an engine torque demand; responsive toselecting the stoichiometric mode, operating the engine in thestoichiometric mode, including operating all engine cylinders at astoichiometric air-fuel ratio (AFR); responsive to selecting the powerEGR mode, operating the engine in the power EGR mode, includingoperating all engine cylinders at the stoichiometric AFR whileincreasing an EGR rate; and responsive to selecting the split lambdamode, operating the engine in the split lambda mode, including operatinga first number of the engine cylinders rich, a second number of theengine cylinders lean, and a third number of the engine cylinders atstoichiometry while providing EGR from the first number of the enginecylinders only.
 12. The method of claim 11, wherein selecting betweenoperating the engine in the stoichiometric mode, the power EGR mode, andthe split lambda mode based on the engine torque demand includes:selecting the stoichiometric mode responsive to the engine torque demandbeing less than a first, lower threshold; selecting the power EGR moderesponsive to the engine torque demand being greater than the firstthreshold and less than a second, higher threshold; and selecting thesplit lambda mode responsive to the engine torque demand being greaterthan the second threshold.
 13. The method of claim 11, wherein operatingthe first number of the engine cylinders rich, the second number of theengine cylinders lean, and the third number of the engine cylinders atstoichiometry includes operating the first number of the enginecylinders at a first, rich AFR, operating the second number of theengine cylinders at a second, lean AFR, and operating the third numberof the engine cylinders at stoichiometry, a degree of enrichment of thefirst, rich AFR equal to a degree of enleanment of the second, lean AFR.14. The method of claim 13, wherein the degree of enrichment isincreased and the EGR rate is decreased as the engine torque demandfurther increases above the second threshold.
 15. The method of claim11, wherein operating the engine in the stoichiometric mode includesincreasing engine torque by increasing an amount of boost provided by aturbocharger, operating the engine in the power EGR mode includesincreasing engine torque by increasing the EGR rate until a thresholdrate is reached while further increasing the amount of boost provided bythe turbocharger, and operating in the split lambda mode includesincreasing engine torque by increasing a degree of enrichment of thefirst half of the engine cylinders while decreasing the EGR rate andstill further increasing the amount of boost provided by theturbocharger.
 16. A system, comprising: an engine, including an oddnumber of cylinders, each cylinder including an exhaust runner coupledto an exhaust manifold; and a controller with computer readableinstructions stored on non-transitory memory that, when executed duringengine operation, cause the controller to: operate a first set ofcylinders at a rich air-fuel ratio, operate a second set of cylinders ata lean air-fuel ratio, and operate a third set of cylinders atstoichiometry responsive to an engine demand greater than an upperthreshold demand, the first set of cylinders including the firstcylinder; and operate the first set of cylinders, the second set ofcylinders, and the third set of cylinders at a same air-fuel ratioresponsive to an engine demand less than the upper threshold demand. 17.The system of claim 16, further comprising an exhaust gas recirculation(EGR) passage coupled between the exhaust runner of a cylinder of thefirst set of cylinders and an intake passage of the engine, and whereinthe controller includes further computer readable instructions stored innon-transitory memory that, when executed during engine operation, causethe controller to: adjust the EGR valve to provide a non-zero amount ofEGR that is less than a threshold amount of EGR while operating thefirst set of cylinders at the rich air-fuel ratio, operating the secondset of cylinders at a lean air-fuel ratio, and operating the third setof cylinders at stoichiometry.
 18. The system of claim 16, furthercomprising an emission control device coupled in an exhaust passagedownstream of the exhaust manifold, and wherein operating the first setof cylinders at the rich air-fuel ratio, operating the second set ofcylinders at a lean air-fuel ratio, and operating the third set ofcylinders at stoichiometry produces a stoichiometric air-fuel ratio atthe emission control device.
 19. The system of claim 18, furthercomprising a turbocharger, including a compressor coupled to an intakeof the engine and a turbine coupled in the exhaust passage upstream ofthe emission control device, and wherein the controller includes furthercomputer readable instructions stored in non-transitory memory that,when executed during engine operation, cause the controller to: increasea boost pressure provided by the turbocharger above atemperature-limited boost pressure threshold while operating the firstset of cylinders at the rich air-fuel ratio, operating the second set ofcylinders at a lean air-fuel ratio, and operating the third set ofcylinders at stoichiometry.
 20. The system of claim 16, furthercomprising a spark plug coupled to each cylinder, and wherein thecontroller includes further computer readable instructions stored innon-transitory memory that, when executed during engine operation, causethe controller to: advance a spark timing of the spark plug coupled toeach cylinder while operating the first set of cylinders at the richair-fuel ratio, operating the second set of cylinders at a lean air-fuelratio, and operating the third set of cylinders at stoichiometry, afirst amount of spark timing advance for the first set of cylindersdifferent than each of a second amount of spark timing advance for thesecond set of cylinders and a third amount of spark timing advance forthe third set of cylinders.